sched: Remove synchronize rcu/sched calls from _cpu_down

There is no need for sync_sched() in _cpu_down as stop_machine()
provides that barrier implicitly. Removing it also helps improve
hot-unplug latency.

The sync_sched/rcu were earlier removed for the same reason by the
commit 9ee349ad ("sched: Fix set_cpu_active() in cpu_down()"),
but recently got added as part of commit <6acce3ef> ("sched:
Remove get_online_cpus() usage.").

CRs-Fixed: 2044149
Change-Id: I97763004454d082d3cc2d9d9dbef7da923608600
Signed-off-by: Kaushal Kumar <kaushalk@codeaurora.org>
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>