Drop leftover gnome-doc-utils build-dependency

Looks like it was forgotten to be removed when updating
to the new upstream release which ported to meson build
system (and at the same time to yelp).

See for example:
commit 3c7a3de1
"Remove uneeded xml2po dep from flatpak-builder manifest."

commit 01c13f72
"Port build system to Meson. Bug 777044."