[Darwin] Remove obsolete OS version checks

The oldest supported deployment target currently is 10.7 [1].  We can
remove a few outdated checks.

[1] https://github.com/llvm/llvm-project/blob/3db893b3712a5cc98ac0dbc88e08df70069be216/compiler-rt/cmake/config-ix.cmake#L397

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D79958