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>

Documentation/admin-guide/sysctl/kernel.rst

@@ -377,13 +377,6 @@ When ``kptr_restrict`` is set to 2, kernel pointers printed using
 %pK will be replaced with 0s regardless of privileges.
 
 
-l2cr (PPC only)
-===============
-
-This flag controls the L2 cache of G3 processor boards. If
-0, the cache is disabled. Enabled if nonzero.
-
-
 modprobe
 ========