llvm-cov: Remove an overly system specific test

It appears that the -filename-equivalence option for testing llvm-cov
doesn't work correctly with -show-expansions. I'm reverting this test
to get the bots green while I look into fixing that.

This partially reverts r217476

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217478 91177308-0d34-0410-b5e6-96231b3b80d8