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.

tests/unit/apps_2.bats

@@ -136,8 +136,6 @@ teardown() {
   assert_success
   run [ -d /home/dokku/great-test-name/tls ]
   assert_failure
-  run [ -f /home/dokku/great-test-name/URLS ]
-  assert_failure
   run [ -f /home/dokku/great-test-name/VHOST ]
   assert_failure
   run /bin/bash -c "curl --silent --write-out '%{http_code}\n' $(dokku url $TEST_APP) | grep 200"