tapdb: remove universe namespace from stats group by

The universe_stats view represents per asset/asset group aggregated
stats. However, the universe namespace field is now specific to a proof
type. This commit removes the namespace field from the stats group by.

tapdb/sqlc/migrations/000007_universe.up.sql

@@ -84,4 +84,4 @@ CREATE VIEW universe_stats AS
         roots.namespace_root
     FROM universe_events u
     JOIN universe_roots roots ON u.universe_root_id = roots.id
-    GROUP BY roots.asset_id, roots.group_key, roots.namespace_root;
+    GROUP BY roots.asset_id, roots.group_key;