[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@362058 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed