Remove implementation of legacy workflows

[sphuber]

Fixes #2378

Note that the database models are kept in place as they will be removed
later, because it will require a migration that will drop the tables.
However, we want to include some functionality that allows the user to
dump the content for storage, before having the migration executed.

[coveralls]

Coverage decreased (-6.5%) to 62.523% when pulling 9e3ccaf13ca275e2a2354607e9d3c1df369182e2 on sphuber:fix_2378_drop_legacy_workflows into 1e339c3 on aiidateam:provenance_redesign.

[coveralls]

Coverage decreased (-0.3%) to 68.692% when pulling 4b75183 on sphuber:fix_2378_drop_legacy_workflows into 1e339c3 on aiidateam:provenance_redesign.

[sphuber]

@giovannipizzi I put the models back so the tests are running now without having to drop the tables. The django test on Jenkins is failing but this I think because it used an old cache of the database of the first run that had the tables dropped. The same test runs fine on Travis.

[giovannipizzi]

I'm not sure why it's reusing the DB, by looking at the code I couldn't understand it. Also it is a bit strange that it's actually triggering the issue during a cascading (we're actually deleting DbNode) - apparently the cascading is done by Django I believe.

Anyway, let's merge - it does feel good, doesn't it? ;-)