[libcxx][libcxxabi] Remove the unused CMake checks

These seemed to have been used in the past but were since removed
by the add_compile_flags_if_supported functions that combine these
these checks and adding the flag, but the original checks were never
removed.

Differential Revision: https://reviews.llvm.org/D62566

llvm-svn: 362058
GitOrigin-RevId: 0528726a69c2f9245645c9efcec2294d2d99b908
1 file changed