[libc++] Removes CMake work-arounds.

CMake older than 3.20.0 is no longer supported.
This removes work-arounds for no longer supported versions.

Reviewed By: #libc, jloser, philnik

Differential Revision: https://reviews.llvm.org/D152099

GitOrigin-RevId: f26b43fa5cc7db4e2533a16c88752f6d9f014c38
1 file changed