cpuidle: menu: Remove single state handling

cpuidle systems where the governor has no choice because there's only
a single idle state are now handled by cpuidle core and bypass the
governor, so remove the related handling.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
[ rjw: Rebase on top of the cpuidle changes merged recently ]
Link: https://patch.msgid.link/20260216185005.1131593-5-aboorvad@linux.ibm.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>