WIP: Remove legacy code for things we do not support

[brycelelbach]

• Remove C++03 codepaths.
• Remove SM < 30.
• Remove GCC < 5 codepaths.
• Remove MSVC < 2017 codepaths.
• Remove Clang < 6 codepaths.
• Remove any other dead code for things we don't support.

[alliepiper]

This had to be the most satisfying pull request you've ever made, eh?

Looked through it and didn't spot any issues. Since it's a sweeping change I want to run this through my MSVC gauntlet and will approve if everything goes well.

[alliepiper]

Builds clean and tests pass with the pushed changes (squash them in if you're good with them)

[alliepiper]

Added another fix-up commit -- some tests were including retag.h.

[alliepiper]

Ran this through my screening tests and found some failures on C++17 configs:

[2/514] Building CXX object CMakeFiles/thrust.cpp.cpp.cpp17.headers.dir/headers/thrust.cpp.cpp.cpp17/advance.h.cpp.o
FAILED: CMakeFiles/thrust.cpp.cpp.cpp17.headers.dir/headers/thrust.cpp.cpp.cpp17/advance.h.cpp.o
/home/av/code/utils/ccache-install/bin/g++-7  -DTHRUST_DEVICE_SYSTEM=THRUST_DEVICE_SYSTEM_CPP -DTHRUST_HOST_SYSTEM=THRUST_HOST_SYSTEM_CPP -DTHRUST_IGNORE_DEPRECATED_CPP_11 -I/home/av/code/src/thrust -Wno-unused-parameter -Werror -Wall -Wextra -Winit-self -Woverloaded-virtual -Wcast-qual -Wno-cast-align -Wno-long-long -Wno-variadic-macros -Wno-unused-function -Wno-unused-variable -Wlogical-op -Wno-noexcept-type -O3 -DNDEBUG   -std=c++1z -MD -MT CMakeFiles/thrust.cpp.cpp.cpp17.headers.dir/headers/thrust.cpp.cpp.cpp17/advance.h.cpp.o -MF CMakeFiles/thrust.cpp.cpp.cpp17.headers.dir/headers/thrust.cpp.cpp.cpp17/advance.h.cpp.o.d -o CMakeFiles/thrust.cpp.cpp.cpp17.headers.dir/headers/thrust.cpp.cpp.cpp17/advance.h.cpp.o -c headers/thrust.cpp.cpp.cpp17/advance.h.cpp
In file included from /home/av/code/src/thrust/thrust/system/detail/generic/advance.inl:19:0,
                 from /home/av/code/src/thrust/thrust/system/detail/generic/advance.h:40,
                 from /home/av/code/src/thrust/thrust/detail/advance.inl:24,
                 from /home/av/code/src/thrust/thrust/advance.h:140,
                 from headers/thrust.cpp.cpp.cpp17/advance.h.cpp:3:
/home/av/code/src/thrust/thrust/iterator/iterator_traits.h:57:1: error: template argument 2 is invalid
 >
 ^
In file included from /home/av/code/src/thrust/thrust/iterator/iterator_traits.h:113:0,
                 from /home/av/code/src/thrust/thrust/system/detail/generic/advance.inl:19,
                 from /home/av/code/src/thrust/thrust/system/detail/generic/advance.h:40,
                 from /home/av/code/src/thrust/thrust/detail/advance.inl:24,
                 from /home/av/code/src/thrust/thrust/advance.h:140,
                 from headers/thrust.cpp.cpp.cpp17/advance.h.cpp:3:
/home/av/code/src/thrust/thrust/iterator/detail/iterator_traits.inl:69:1: error: template argument 2 is invalid
 >
 ^
/home/av/code/src/thrust/thrust/iterator/detail/iterator_traits.inl:84:42: error: ‘type’ in ‘struct thrust::iterator_system<int*>’ does not name a type
   typedef thrust::iterator_system<int*>::type type;
                                          ^~~~
/home/av/code/src/thrust/thrust/iterator/detail/iterator_traits.inl:90:48: error: ‘type’ in ‘struct thrust::iterator_system<const int*>’ does not name a type
   typedef thrust::iterator_system<const int*>::type type;

It looks like it is only on the non-cuda builds:

Building thrust.cpp.cpp.cpp11...   Walltime:   0:12.19 |    40 CPU secs | ExitCode: 0
Building thrust.cpp.cpp.cpp14...   Walltime:   0:03.68 |    34 CPU secs | ExitCode: 0
Building thrust.cpp.cpp.cpp17...   Walltime:   0:01.09 |     6 CPU secs | ExitCode: 1
Building thrust.cpp.omp.cpp11...   Walltime:   0:03.83 |    35 CPU secs | ExitCode: 0
Building thrust.cpp.omp.cpp14...   Walltime:   0:03.88 |    36 CPU secs | ExitCode: 0
Building thrust.cpp.omp.cpp17...   Walltime:   0:01.03 |     6 CPU secs | ExitCode: 1
Building thrust.cpp.tbb.cpp11...   Walltime:   0:03.85 |    35 CPU secs | ExitCode: 0
Building thrust.cpp.tbb.cpp14...   Walltime:   0:03.85 |    35 CPU secs | ExitCode: 0
Building thrust.cpp.tbb.cpp17...   Walltime:   0:01.18 |     6 CPU secs | ExitCode: 1
Building thrust.omp.omp.cpp11...   Walltime:   0:03.88 |    36 CPU secs | ExitCode: 0
Building thrust.omp.omp.cpp14...   Walltime:   0:03.86 |    35 CPU secs | ExitCode: 0
Building thrust.omp.omp.cpp17...   Walltime:   0:01.02 |     6 CPU secs | ExitCode: 1
Building thrust.tbb.tbb.cpp11...   Walltime:   0:04.16 |    38 CPU secs | ExitCode: 0
Building thrust.tbb.tbb.cpp14...   Walltime:   0:04.17 |    39 CPU secs | ExitCode: 0
Building thrust.tbb.tbb.cpp17...   Walltime:   0:01.21 |     6 CPU secs | ExitCode: 1
Building thrust.cpp.cuda.cpp11...  Walltime:   0:06.42 |    57 CPU secs | ExitCode: 0
Building thrust.cpp.cuda.cpp14...  Walltime:   0:06.81 |    56 CPU secs | ExitCode: 0
Building thrust.cpp.cuda.cpp17...  Walltime:   0:07.87 |    56 CPU secs | ExitCode: 0