sched/core: Drop a preempt_disable_rt() statement

[ Upstream commit 761126efdcbe3fa3e99c9079fa0ad6eca2f251f2 ]

The caller holds a lock which already disables preemption.
Drop the preempt_disable_rt() statement in get_nohz_timer_target().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>