fix(sql): don't render asset in collateral utxo output

Correct asset is part of normal output. The asset in output collateral
is completely wrong and it should only contain ADA value. This aligns
it with input UTXOs.

Closes #160

src/sql/txs/txs_hash_utxos_2.sql

@@ -30,20 +30,7 @@ FROM (
     UNION ALL
     SELECT txo.address AS "address",
       txo.value::TEXT AS "amount_lovelace",
-      -- cast to TEXT to avoid number overflow
-      (
-        SELECT json_agg(
-            json_build_object(
-              'unit',
-              CONCAT(encode(policy, 'hex'), encode(name, 'hex')),
-              'quantity',
-              mto.quantity::TEXT -- cast to TEXT to avoid number overflow
-            )
-          )
-        FROM ma_tx_out mto
-          JOIN multi_asset ma ON (mto.ident = ma.id)
-        WHERE mto.tx_out_id = txo.id
-      ) AS "amount",
+      null AS "amount",
       encode(txo.data_hash, 'hex') AS "data_hash",
       encode(dat.bytes, 'hex') AS "inline_datum",
       true AS "collateral",