m68k: is_mem is useless

Remove is_mem as it is never tested anymore since:

    commit bfa50bc2
    Author: aliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162>
    Date:   Tue Nov 18 20:26:41 2008 +0000

        Remove premature memop TB terminations (Jan Kiszka)

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>