[clang] Remove a FIXME that we can't fix

I added this recently, but it looks like several tests very intentionally
check that `-mios-version-min=foo --target=x86_64-apple-ios` does simulator
builds. So we can't easily remove this hack, even though it makes little
sense in an arm mac world. (Here, you _have_ to say
`-mios-simulator-version-min=` or `--target=arm64-apple-ios-simulator`.)

The tests that check this:
  Clang :: Driver/darwin-ld.c
  Clang :: Driver/darwin-simulator-macro.c
  Clang :: Driver/darwin-version.c

No behavior change.

Differential Revision: https://reviews.llvm.org/D132400