docs: drop l2cr from sysctl/kernel.rst

The l2cr sysctl entry was removed in commit c2f3dabe

 ("sysctl:
kill binary sysctl KERN_PPC_L2CR"), this removes the corresponding
documentation.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>