remove obsolete helpers

The package helpers may no longer be necessary:

• make-abiword: bionic already includes flickering patch
• make-dasher: gnome-speech is no longer a dependency
• make-db: source package no longer exists upstream
• make-file-roller: packagekit support disabled since 2.30 (see changelog)
• make-gnome-control-center: all modified files no longer exist upstream
• make-gnome-panel: replaced by mate-panel in trisquel
• make-picard: Musicbrainz explicitly requires plugins to be GPLv2-compatible, so their repo is not a FSDG issue