[lld-macho][nfc] Avoid using std::map for PlatformKinds

The mappings we were using had a small number of keys, so a vector is
probably better. This allows us to remove the last usage of std::map in
our codebase.

I also used `removeSimulator` to simplify the code a bit further.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D105786