PM / AVS: SmartReflex: remove unused function

omap_sr_register_pmic() was introduced in 2010 in commit

	984aa6db ("OMAP3: PM: Adding smartreflex driver support.")

. There was never any caller of this function in mainline resulting in a
warning

	sr_init: No PMIC hook to init smartreflex

for each machine where this driver is enabled. So remove the unused
function and the pr_warn.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>