[libc++] Remove some workarounds for unsupported GCC and Clang versions

There is a lot more we can do, in particular in <type_traits>, but this
removes some workarounds that were gated on checking a specific compiler
version.

Differential Revision: https://reviews.llvm.org/D108923

GitOrigin-RevId: a4cb5aefd545104e52139d9c3ba6c51165a8dabe
1 file changed