perf(db): Remove obsolete indexes (#1376)

## What ❔

Migration that removes `transactions_contract_address_idx`,
`events_tx_initiator_address`

## Why ❔

They are not used

## Checklist

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- [ ] Tests for the changes have been added / updated.
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core/lib/dal/migrations/20240306155535_remove_obsolete_indexes.down.sql

@@ -0,0 +1,4 @@
+CREATE INDEX IF NOT EXISTS events_tx_initiator_address_idx
+    ON events (tx_initiator_address);
+CREATE INDEX IF NOT EXISTS transactions_contract_address_idx
+    ON transactions (contract_address);

core/lib/dal/migrations/20240306155535_remove_obsolete_indexes.up.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS transactions_contract_address_idx;
+DROP INDEX IF EXISTS events_tx_initiator_address;