grep: remove local option --only-files permanently

grep: remove local option --only-files permanently

The grep option --only-files exists only on DragonFly.  It was added in
late 2005 by Simon Schubert as a quick and dirty way to search object
directories using grep rather than a complex find command.  The issue
was that some directory symlinks were created that produced recursive
loops.  There wasn't a lot of support for the suggestion, but when the
code was altered to allow non-directory symlinks to be followed by the
--only-files option, the main objection was addressed.

The problem is that this modification is making it difficult to maintain
grep.  The code that it touches changes frequently, and its unmergable
with grep 2.12.  The burden of maintaining this option is too much
considering it may only have a single user.

Discussed with: Dillon

For the purposes of merging grep 2.12, the hammer bug fix is also being
removed since it doesn't merge cleanly, but it will be restored after
the merge.