context_tracking: Interrupts always disabled for ct_idle_exit()

Now that the idle-loop cleanups have ensured that rcu_idle_exit() is
always invoked with interrupts disabled, remove the interrupt disabling
in favor of a debug check.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>