[dataimport] remove drop_index parameter from massive store "drop_index=False" also implied not dropping any constraints, in particular foreign keys, which meant any attempt to import entities would fail, because we only add metadata (the entities table) after the entity's insertion.

[dataimport/test] update call to ucsvreader Silences warning: [3.20] 'separator' kwarg is deprecated, use 'delimiter' instead

[dataimport] remove deprecated methods from massive store This code is new in cubicweb, no reason for it to carry deprecated stuff from the start.

[dataimport/test] use store.finish instead of deprecated store.cleanup

[dataimport] drop no more used parameter on massive store

[dataimport] methods that modify in-place shouldn't return value Also mark those methods private and drop some useless comments in passing. Related to #5414760

[dataimport] implement new store API on massive store and deprecate the old one. Related to #5414760