Eliminate temporary BC layers from rewrite
Eliminate temporary BC layers from rewrite

During the frontend rewrite, we introduced mithril patches for a `route` attr, and for `m.stream`. Later, we decided not to go that route, but not to remove the patches yet to avoid breaking extensions while we were finalizing the replacements. We can now remove these.

Other BC layers are for things from before beta 14, so those remain in place.

flarum/core,flarum/core,flarum/core