No need to store and use boards

[Christian-B]

Follow up to #350

BMPConnectionData is always created with boards=[0]

So remove that altogether

Including its usage in https://github.com/SpiNNakerManchester/SpiNNMan/blob/master/spinnman/messages/scp/impl/set_power.py

[rowleya]

I guess that is because we 1) never use a multi-board machine without spalloc and 2) spalloc is now in Java. I guess if that is the recommended use, we may as well make that a non parameter.

[Christian-B]

Likely to be removed but I suggest leaving until spin 2 fully implemented,