Ballista should not have separate DataFrame implementation

[andygrove]

hen building the Ballista POC it was necessary to implement a new DataFrame API that wrapped the DataFusion API.

One issue is that it wasn't possible to override the behavior of the collect method to make it use the Ballista context rather than the DataFusion context.

Now that the projects are in the same repo it should be easier to fix this and have users always use the DataFusion DataFrame API.