The package is no longer useful

This reverts commit bd974458.

I didn't realize there is already a helper make-gnome-colors.
Helpers need to have the source package's name, in this case
gnome-colors-common is a binary package prooduced by source
package gnome-colors.

I'm reverting this, and manually merging the code from both helpers.

shim-signed is not avaliable in Trisquel