[libc++] Remove old CMake workaround

We haven't had any complaints so far, and I don't think anybody builds
libc++ from source for that old platform anymore.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@355336 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed