Revert "checkpatch: Error out if deprecated RCU API used"

The definition for single-argument kfree_rcu() has been removed,
so that any further attempt to use it will result in a build error.
Because of this build error, there is no longer any need for a special
check in checkpatch.pl.

Therefore, revert commit 1eacac32.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>