Remove use of obsolete preprocessor symbols

[idg10]

As https://github.com/dotnet/reactive/blob/main/Rx.NET/Documentation/adr/0001-net7.0-era-tooling-update.md#out-of-support-tfms-1 describes, there are numerous places in the code that use conditional compilation with preprocessor constants that are no longer relevant. These sections were added back when we targetted older runtimes and lower versions of .NET Standard.

They now serve no purpose, and just add complication and clutter to the build. We should remove all conditional compilation based on these now-irrelevant symbols.