docs: remove .toc-title class

docs: remove .toc-title class

The "toc-title" class was introduced in commit ef88f10eb877
("[media] doc-rst: backward compatibility with older Sphinx versions")
as a workaround for Sphinx versions older than 1.4.x. However, these
old versions have been deprecated since commit 31abfdda6527
("docs: Deprecate use of Sphinx < 2.4.x").

Having now changed all the toc-title users to use the :caption:
attribute, we can also remove the custom style.

Note that the toc-title class is separate from the "kernel-toc"
logic that was introduced in commit c404f5d4f099 ("docs: Add more
information to the HTML sidebar").

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-12-vegard.nossum@oracle.com