Lookup namespace by pages host using find_by_path

In `Namespace#find_by_pages_host` we always have top level namespaces
which means we can use `find_by_path` instead of `find_by_full_path` and
avoid join on `routes`.

Related to https://gitlab.com/gitlab-org/gitlab/issues/207096.

app/models/namespace.rb

@@ -129,7 +129,7 @@ class Namespace < ApplicationRecord
       return unless host.ends_with?(gitlab_host)
 
       name = host.delete_suffix(gitlab_host)
-      Namespace.find_by_full_path(name)
+      Namespace.find_by_path(name)
     end
 
     # overridden in ee