Remove duplicate 'select' database statement

The `select` method has the same definition in almost all database adapters, so it can be moved from the database-specific adapters (PostgreSQl, MySQL, SQLite) to the abstract `database_statement`:

```ruby
def select(sql, name = nil, binds = [])
  exec_query(sql, name, binds)
end
```

---

More details about this commit: the only two DB-specific adapters that have a different definition of `select` are MySQLAdapter and MySQL2Adapter.

In MySQLAdapter, `select` invokes `exec_query(sql, name, binds)`, so calling `super` achieves the same goal with less repetition.

In MySQL2Adapter, `select` invokes `exec_query(sql, name)`, that is, it does not pass the `binds` parameter like other methods do. However, MySQL2Adapter's `exec_query` works exactly the same whether this parameters is passed or not, so the output does not change. Here is [MySQL2Adapter's `exec_query`](https://github.com/rails/rails/blob/74a527cc63ef56f3d0a42cf638299958dc7cb08c/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb#L228L231):

```ruby
def exec_query(sql, name = 'SQL', binds = [])
  result = execute(sql, name)
  ActiveRecord::Result.new(result.fields, result.to_a)
end
```