sched: kill sync_cpu maintenance

We assume boot CPU as a sync CPU and initialize it's window_start
to sched_ktime_clock(). As windows are synchronized across all
CPUs, the secondary CPUs' window_start are initialized from the
sync_cpu's window_start. A CPU's window_start is never reset, so
this synchronization happens only once for a given CPU. Given this
fact, there is no need to reassigning the sync_cpu role to another
CPU when the boot CPU is going offline. Remove this unnecessary
maintenance of sync_cpu and use any online CPU's window_start as
reference.

Change-Id: I169a8e80573c6dbcb1edeab0659c07c17102f4c9
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
[rameezmustafa@codeaurora.org: Port to msm-4.9]