[lld-macho][nfc] InputSections don't need to track their total # of callsites

... only whether they have more than zero. This simplifies the code slightly.

I've also moved the field into the ConcatInputSection subclass since it doesn't
actually get used by the other InputSections.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D115539