srcu: Remove redundant GP sequence checks in srcu_funnel_gp_start

We will perform GP sequence checking at the beginning of srcu_gp_start,
thus making it safe to remove duplicate GP sequence checks prior to
calling srcu_gp_start.

Signed-off-by: Feng Lee <379943137@qq.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>