locks: Remove unnecessary IS_POSIX test

Since following change

commit bd61e0a9
Author: Jeff Layton <jlayton@primarydata.com>
Date:   Fri Jan 16 15:05:55 2015 -0500

    locks: convert posix locks to file_lock_context

all Posix locks are kept on their a separate list, so the test is
redudant.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: Jeff Layton <jlayton@primarydata.com>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>