ARM: shmobile: cpuidle: remove useless WFI function

Remove the shmobile_enter_wfi function which is the same as the
common WFI enter function from the arm cpuidle driver defined
with the ARM_CPUIDLE_WFI_STATE macro.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>