[libc++abi] Remove unnecessary atomic_support.h header from libc++abi

The file was a duplicate of atomic_support.h in libc++. Since we now
require the libc++ sources in order to build libc++abi, it's OK to
remove this duplication.

Thanks to @chandlerc for noticing this.

Differential Revision: https://reviews.llvm.org/D110103

GitOrigin-RevId: f8b1cc36578608f2f840d7ea8b0aa9b28fdaa492
8 files changed