Remove doclifter "signature" from Docbook man pages' source

Remove doclifter "signature" from Docbook man pages' source

This no longer serves any purpose, and might mislead contributors
into thinking that this XML is not the source for the man pages.
(The man(7)-formatted man pages used to be the canonical source for
the XML, but now it's the other way round.)

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=59805
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>