[Coverage] Delete getCounterMismatches, it's dead code (NFC)

Exactly one counted region is inserted into a function record for every
region in a coverage mapping.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339193 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed