cpuidle: Remove unnecessary wrapper cpuidle_get_last_residency()

cpuidle_get_last_residency() is just a wrapper for retrieving
the last_residency member of struct cpuidle_device.  It's also
weirdly the only wrapper function for accessing cpuidle_* struct
member (by my best guess is it could be a leftover from v2.x).

Anyhow, since the only two users (the ladder and menu governors)
can access dev->last_residency directly, and it's more intuitive to
do it that way, let's just get rid of the wrapper.

This patch tidies up CPU idle code a bit without functional changes.

Signed-off-by: Fieah Lim <kw@fieahl.im>
[ rjw: Changelog cleanup ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>