Cut noexcept calculations in dynamic-inl.h

Summary:
- GCC 4.9 previously required marking `TypeError` special member
  functions with conditional noexcept.
- GCC 5.1 does not have this issue. So, remove the conditional noexcept
  as they always evalaute to true in the existing code.

folly/dynamic-inl.h

@@ -134,16 +134,10 @@ struct FOLLY_EXPORT TypeError : std::runtime_error {
       const std::string& expected,
       dynamic::Type actual1,
       dynamic::Type actual2);
-  // TODO: noexcept calculation required through gcc-v4.9; remove once upgrading
-  // to gcc-v5.
-  TypeError(const TypeError&) noexcept(
-      std::is_nothrow_copy_constructible<std::runtime_error>::value);
-  TypeError& operator=(const TypeError&) noexcept(
-      std::is_nothrow_copy_assignable<std::runtime_error>::value);
-  TypeError(TypeError&&) noexcept(
-      std::is_nothrow_move_constructible<std::runtime_error>::value);
-  TypeError& operator=(TypeError&&) noexcept(
-      std::is_nothrow_move_assignable<std::runtime_error>::value);
+  TypeError(const TypeError&) noexcept;
+  TypeError& operator=(const TypeError&) noexcept;
+  TypeError(TypeError&&) noexcept;
+  TypeError& operator=(TypeError&&) noexcept;
   ~TypeError() override;
 };
 

folly/dynamic.cpp

@@ -63,14 +63,10 @@ TypeError::TypeError(
           dynamic::typeName(actual1),
           dynamic::typeName(actual2))) {}
 
-TypeError::TypeError(const TypeError&) noexcept(
-    std::is_nothrow_copy_constructible<std::runtime_error>::value) = default;
-TypeError& TypeError::operator=(const TypeError&) noexcept(
-    std::is_nothrow_copy_assignable<std::runtime_error>::value) = default;
-TypeError::TypeError(TypeError&&) noexcept(
-    std::is_nothrow_move_constructible<std::runtime_error>::value) = default;
-TypeError& TypeError::operator=(TypeError&&) noexcept(
-    std::is_nothrow_move_assignable<std::runtime_error>::value) = default;
+TypeError::TypeError(const TypeError&) noexcept = default;
+TypeError& TypeError::operator=(const TypeError&) noexcept = default;
+TypeError::TypeError(TypeError&&) noexcept = default;
+TypeError& TypeError::operator=(TypeError&&) noexcept = default;
 TypeError::~TypeError() = default;
 
 // This is a higher-order preprocessor macro to aid going from runtime