Stop tracking stat(2).

None of lstat(2), fstat(2), fstatat(2) were tracked either.

The other filemon implementations also do not track stat(2), nor
does bmake utilize it. The act of opening a file for read should
be enough to decide that a file is a dependency. There could be
rare cases where just having a file would cause a dependency but it
is unlikely.

MFC after: 2 weeks
Also noted by: sjg
Sponsored by: EMC / Isilon Storage Division