Remove -lower-global-dtors-via-cxa-atexit flag

Remove the `-lower-global-dtors-via-cxa-atexit` escape hatch introduced
in D121736 [1], which switched the default lowering of global
destructors on MachO to use `__cxa_atexit()` to avoid emitting
deprecated `__mod_term_func` sections.

I added this flag as an escape hatch in case the switch causes any
problems.  We didn't discover any problems so now we can remove it.

[1] https://reviews.llvm.org/D121736

rdar://90277838

Differential Revision: https://reviews.llvm.org/D145715