Remove commented test code that will never be restored
Merge test-graph.ML into test-ograph.ML
Re-organise ograph signature tests

All functions are either tested or have a TODO in the test file.  The
tests are in the same order as the signature.
Remove deprecated code

This was all already commented-out
simplified_bang_graph.ML -> bang_graph.ML
do_plug (etc) -> plug_anon (etc)