x86/idle: Remove is_idle flag

Upon removal of the idle_notifier, all accesses to the "is_idle" flag serve
no purpose.

Signed-off-by: Len Brown <len.brown@intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/e4a24197cf9c227fcd1ca2df09999eaec9052f49.1479449716.git.len.brown@intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>