[libc++] Remove Lit annotations for unsupported GCC versions from the test suite

Since we officially don't support several older compilers now, we can
drop a lot of the markup in the test suite. This helps keep the test
suite simple and makes sure that UNSUPPORTED annotations don't rot.

This is the first patch of a series that will remove annotations for
compilers that are now unsupported.

Differential Revision: https://reviews.llvm.org/D107787