[ENH] reduce legacy logic in test framework and refactor to scenarios

[fkiraly]

This PR reduces the amount of legacy logic by removing a number of old functions, and refactoring some content to the scenarios framework.

• removes _make_args logic and replaces by direct calls to fixture generators in sporadic instances where it was still called. The logic is legacy and has been mostly replaced by scenarios.
• replaces a leftover transformer test for numpy3D mtype input compatibility by a transformer scenario which uses numpy. The old test was in test_all_panel_transformers, the new scenario is TransformerFitTransformPanelUnivariateNumpyWithClassYOnlyFit
• removes three of the four old transformer mixins from the test framework: _PanelToTabularTransformer, _SeriesToPrimitivesTransformer, _SeriesToSeriesTransformer. The _PanelToPanelTransformer is still in use in some transformers. The other mixins are no longer in use and replaced by transformer tags since a while already.
• removes the shapelet transformers from _contrib. The shapelet transform has been migrated to the transformations module already, and the one diff - the contracted version - seemed buggy/incomplete.