Revert "Add schema for upcoming changes to how applets are loaded and handled"

This reverts commit 0ceeeefefde38a0ebe2b9fc44a4d95260ae6fe14.
Due the GSettings issue mentioned in ba370b0fb049ff9d334a86fcfaf57bfce48930b9
we currently cannot sanely use relocatable schemas for budgie-panel applets.
As and when the API is complete in GLib, this will be restored.