rcu: Remove redundant CONFIG_PROVE_RCU #if condition

The #if condition controlling the rcu_preempt_sleep_check() definition
has a redundant check for CONFIG_PREEMPT_RCU, which is already checked
for by an enclosing #ifndef.  This commit therefore removes this redundant
condition from the inner #if.

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