[Indexer/Schema] Remove query cost function

## Description

We no longer use the query_cost function anywhere (it has been
replaced by a diesel extension that enables us to call `EXPLAIN` on a
query), so cleaning it up to avoid having to maintain it (and create a
MySQL compatible version of it).

## Test plan

```
sui-indexer$ diesel setup --database-url=$DB --migration-dir='migrations/mysql'
sui-indexer$ diesel database reset --database-url=$DB --migration-dir='migrations/mysql'
sui$ cargo nextest run -p sui-indexer
sui$ cargo nextest run -p sui-graphql-rpc
sui$ cargo nextest run -p sui-graphql-e2e-tests --features pg_integration
```