docs: kernel-doc: Get rid of xml_escape() and friends

XML escaping is a worry that came with DocBook, which we no longer have any
dealings with.  So get rid of the useless xml_escape()/xml_unescape()
functions.  No change to the generated output.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>