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

@@ -80,8 +80,7 @@ CREATE VIEW universe_stats AS
         COUNT(CASE WHEN u.event_type = 'SYNC' THEN 1 ELSE NULL END) AS total_asset_syncs,
         COUNT(CASE WHEN u.event_type = 'NEW_PROOF' THEN 1 ELSE NULL END) AS total_asset_proofs,
         roots.asset_id,
-        roots.group_key,
-        roots.namespace_root
+        roots.group_key
     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;