futex: Cleanup generic SMP variant of arch_futex_atomic_op_inuser()

The generic SMP variant of arch_futex_atomic_op_inuser() returns always
-ENOSYS so the switch case and surrounding code are pointless. Remove it
and just return -ENOSYS.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/12bdaca8-99eb-e576-f842-5970ab1d6a92@virtuozzo.com