hotdoc: remove --gtk-doc-remove-xml

This argument is deprecated, and as we don't document
sources generated by gdbus-codegen, also was useless,
and prevented us from using raw html in our documentation.

Signed-off-by: Mathieu Duponchelle <mathieu.duponchelle@opencreed.com>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4501