Fixes T11285. We can end up loading panel handles while processing edits (e.g., disabling menu items on a project). We probably started loading these after the modular transaction changes in T9789, which load the handle for the transaction object unconditionally.
The handles aren't too useful, but they currently fail to load/build because panels don't have a URI. We could give them some sort of method here, but just nuke it for now since they don't appear anywhere and this unclogs the daemon queue.