checkpatch: Remove SRCU-lite deprecation

Now that SRCU-lite has been removed from the kernel, let's remove the
now-redundant deprecation from checkpatch.pl.

Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>