git-rm: do not fail on already removed file.

Often the user would do "/bin/rm foo" before telling git, but
then want to tell git about it.  "git rm foo" however would fail
because it cannot unlink(2) foo.

Treat ENOENT error return from unlink(2) as if a successful
removal happened.

Signed-off-by: Junio C Hamano <junkio@cox.net>