Delete DataSource object

[FelixMalfait]

The dataSource object is just used to do a 1:1 mapping between a workspace and its schema.
We thought it'd be useful for remote objects but it won't be and the current setup is therefore confusing.

Option 1

• Delete DataSource object
• Create a schema column on the workspaces table

Option 2

• Drop most columns on dataSource (no need for type, isRemote...)

I have a preference for option (1)