refactor: simplify how url generation works

The URLS file is no longer necessary as urls are now generated directly from the app VHOST file.

This also moves all the url generation logic to the domains plugin.

plugins/domains/install

@@ -10,6 +10,7 @@ trigger-domains-install() {
   shopt -s nullglob
   for app in $(dokku_apps "false" 2>/dev/null); do
     domains_setup "$app"
+    rm -f "$DOKKU_ROOT/$APP/URLS"
   done
 }