Remove ModelGroups from Analysis Projects

This is a proposal to remove the the automatically generated ModelGroups for AnalysisProjects.

The generation of one ModelGroup per AnP predates the direct bridge table that we have now. This bridge table is the canonical source of AnP membership and (unlike ModelGroups) is not mutable by end users.

That leaves two use cases for these auto generated model groups that I can think of:

* Getting an at-a-glance overview of model status for the whole project. I think this has been superseded by @mark-burnett's `genome analysis-project view` command and the forthcoming AnP view in Spectacle. Is anyone actually loading up these model groups in imp or at the cli currently?

* As inputs to commands that take model groups. I don't think that this works anyways as AnP model groups are heterogeneous (potentially containing RnaSeq, QC, RefAlign, SomVal, etc) and I'm unaware of any commands that take such model groups.

Note that this would not remove the auto-generated `Genome::Project` based model groups that roughly correspond to LIMS work orders. I believe they are still in use.

I think that @jasonwalker80 and @ernfrid  are the two likely users of these, so if one of you wants to chime in, it'd be much appreciated.

EDIT: If I get the go-ahead on this, I will also append a sqitch migration to this PR that removes the appropriate column. 
This should have a return value regardless of the value of `$created_new`.