[Darwin] Remove workaround for symbolication in iOS simulator runtimes

A while ago we added some code to the sanitizer runtimes for iOS
simulators to allow `atos` (external process) to inspect the sanitized
process during report generation to enable symbolication.  This was done
by setting the `__check_mach_ports_lookup` env var early during process
startup which came with a couple of complications.

This workaround is not required anymore and removing it fixes TSan in
the iOS simulator after the new TSan runtime landed.
(https://reviews.llvm.org/D112603)

Relevant/reverted revisions:
https://reviews.llvm.org/D78178
https://reviews.llvm.org/D78179
https://reviews.llvm.org/D78525

rdar://86472733

Differential Revision: https://reviews.llvm.org/D115767