ARM: shmobile: cpuidle: remove shmobile_enter_wfi prototype

Commit 688036b538974de32ce55be8b0e013b003992abc removed the function
'shmobile_enter_wfi' but we forgot to remove the definition in the header file.

Note this function is just an alias to 'cpu_do_idle()' wrapped into a cpuidle
function callback prototype which already exists with the default WFI state
and the arm_simple_enter function.

Remove the function prototype.

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>