kernel-doc: Don't try to mark up function names

We now have better automarkup in sphinx itself and, besides, this markup
was incorrect and left :c:func: gunk in the processed docs.  Sort of
discouraging that nobody ever noticed...:)

As a first step toward the removal of impenetrable regex magic from
kernel-doc it's a tiny one, but you have to start somewhere.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>