timer: Remove a BUG_ON check in migrates timers

__migrate_timers() can be called from both hotplug and
core isolation contexts. When isolatoins is happening,
the other core may be processing the timers simultaniously,
So remove a BUG_ON(old_base->running_timer) check in
__migrate_timers() function.

Change-Id: I6483fe0d95d0ce57d9f8d478ac9df64d47bff0fe
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>