docs: remove kernel-doc.pl

We've been using the Python version and nobody has missed this one.  All
credit goes to Mauro Carvalho Chehab for creating the replacement.

Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>