[Coverage] Remove redundant handleFileExit() call (NFC)

I added this call in r271308. It's redundant because it's dominated by a
call to extendRegion().

Thanks to Justin Bogner for pointing this out!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271331 91177308-0d34-0410-b5e6-96231b3b80d8