rcuscale: Remove redundant WARN_ON_ONCE() splat

There are two places where WARN_ON_ONCE() is called two times
in the error paths. One which is encapsulated into if() condition
and another one, which is unnecessary, is placed in the brackets.

Remove an extra WARN_ON_ONCE() splat which is in brackets.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>