removed remote unit ids var from Relation

[PietroPasotti]

removed redundant remote unit ids (alternative constructor for remote relation data), legacy

before:
you could initialize a Relation with three remote units (empty databags) equivalently in one of two ways:

• by passing remote_unit_ids=[1,3,5]
• by passing remote_units_data={1:{}, 3:{}, 5:{}}

now:
remote_unit_ids is gone; only remote_units_data={1:{}, 3:{}, 5:{}} remains.

Why:

• simpler API
• simpler to determine when a relation is consistent (what if a user passes both?)
• simpler to use the .replace() API: you don't have to replace both every time.