Resolves fdo#87016 Kill "Use system font for user interface" for good

Change-Id: Ica648d4a49ef4f36fdab557a98e7df9d31a3a240
Reviewed-on: https://gerrit.libreoffice.org/13311
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>