API Revert introduction of SQLSelect

In the ORM rewrite in 3.2 the `SQLSelect` class was introduced, intended to supersede `SQLQuery`. However, this introduced several breakages in the api, notably any extension which overrode the `DataExtension::augmentSQL` method.

In order to better promote ease of upgrade between 3.1 and 3.2 I've instead substituted SQLQuery back in again for SQLSelect and removed references to this new class. SQLDelete SQLUpdate and SQLInsert still exist along side this as they did before, and function in the same way.

This should allow most code using SQLQuery directly to continue working, with far fewer upgrade changes necessary.

R.I.P. `SQLSelect`. We hardly knew you, before you were torn from us.