[Modules] Remove darwin specific code to check for SystemVersion.plist

This isn't need anymore and modules options -fbuild-session-file and
-fmodules-validate-once-per-build-session already provide a sane
mechanism to validate the system headers.

rdar://problem/19767523

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300027 91177308-0d34-0410-b5e6-96231b3b80d8