rcu/sync: remove un-used rcu_sync_enter_start function

With commit '6a010a49

 ("cgroup: Make !percpu threadgroup_rwsem
operations optional")' usage of rcu_sync_enter_start is removed.

So this function can also be removed.

In the words of Oleg Nesterov:

	__rcu_sync_enter(wait => false) is a better alternative if
	someone needs rcu_sync_enter_start() again.

Link: https://lore.kernel.org/all/20220725121208.GB28662@redhat.com/
Signed-off-by: Onkarnath <onkarnath.1@samsung.com>
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>