Revert "PECOFF: Temporarily add a lock to un-break buildbot."

This reverts commit r230086. I added a lock to guard FileCOFF::doParse(),
which killed parallel file parsing. Now the buildbots got back to green,
I believe the threading issue was resolved, so it's time to remove the
guard to see if it works with the buildbots.

llvm-svn: 230886