posix-timers: Remove mmtimer leftovers

After removing mmtimer, the mmtimer struct can be removed from the k_itimer
struct.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Russ Anderson <rja@sgi.com>
Cc: Dimitri Sivanich <sivanich@sgi.com>
Cc: Mike Travis <mike.travis@hpe.com>
Cc: Nate Zimmer <nzimmer@sgi.com>
Cc: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/20170526130534.GE30788@hpe.com