cpuidle: Remove pointless stub

The cpuidle.h header is declaring a function with an empty stub
for the cpuidle disabled case, but that function is only called
by cpuidle governors which depend on cpuidle anyway.

In other words, the function is only called when cpuidle is enabled,
so there is no need for the stub.

Remove the pointless stub.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>