libdnf: remove non-functional gtk-doc support

libdnf: remove non-functional gtk-doc support

The gtk-doc integration doesn't actually work, as it isn't built by
default and the targets need to be triggered manually.

As clearly nobody was using it, and upstream appears to prefer doxygen
for API documentation, remove the gtk-doc support.

This also means adding gettext to the dependencies, as this was being
pulled in via gtk-doc before but is actually needed for the translations.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>