Documentation: kernel-parameters.txt remove capability.disable

Remove the documentation for capability.disable.  The code supporting
this parameter was removed with commit 5915eb53

 ("security: remove
dummy module")

Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Cc: Rob Landley <rob@landley.net>
Cc: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>