cpuidle: unexport tick_nohz_get_sleep_length

This patch removes the unused
EXPORT_SYMBOL_GPL(tick_nohz_get_sleep_length),
which we no long user b/c we no longer build optional modules.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Len Brown <len.brown@intel.com>