docs: kerneldoc-preamble.sty: Remove code for Sphinx <2.4

Now that Sphinx 2.4.4 or better is required, get rid of
\providecommand{}'s for compatibility with Sphinx 1.7.9.

While at it, reword the comment on \sphinxtableofcontentshook
for better description of why it needs to be emptied.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <ed1ec6f2-0050-46f6-807d-8679f26427e9@gmail.com>