Properly highlight nested comments in vim

Properly highlight nested comments in vim

Prior to this commit, every block comment /* */ required two closing tags for
every opening tag in order to terminate the highlighting. Setting and testing
for a variable was the culprit, though I'm not certain why, but they appear to
just be boilerplate lines from whatever pcwalton based this file upon. I've
looked at other officially-distributed vim highlighting files and none seem to
do the test that this commit removes, so I'm fairly certain it didn't provide
anything vital. And now comment highlighting works!