cifs: remove unused function cifs_oplock_break_wait

Commit 74316201 ("sched: Remove proliferation of wait_on_bit() action
functions") has removed the call to cifs_oplock_break_wait, making this
function unused; remove it.

This fixes the following compilation warning:

  fs/cifs/misc.c:578:1: warning: ‘cifs_oplock_break_wait’ defined but not used [-Wunused-function]

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Steve French <sfrench@samba.org>
Signed-off-by: Steve French <smfrench@gmail.com>