Delete `flatten_networks`

[mtfishman]

I don't think flatten_networks is needed now that we have some better concepts like forms, partitioned graphs, etc.

@JoeyT1994 seems like you already started removing it's usage in the tests in #162?

[JoeyT1994]

Yeah I have been moving away from it.

I think, with the changes in #162, the only use I can see of it is in test_contraction_sequence_to_graph which was some code from a while back converting contraction_sequences to a graphical representation.