cad/oregano: stop using rarian and small cleanup

rarian is deprecated and unused anymore, stop teaching oregano package
to run scrollkeeper on those files.

While here stop installing the compiled glib schema which will overwrite
the system one.

Oregano uses waf, which will wrongly run glib-compile-schema, but also
update-mime-info, update-desktop-database which should be run at install
time only.

A real fix would be to prevent waf from running all those programs, but
waf is a blackbox, and patching it is complicated per package.