3.0 Deprecations: Remove deprecation warnings from featurizers

[ancalita]

Context:
In preparation for the 3.0 release, we need to remove all deprecated code/warnings.

These deprecation warnings are being raised in:

• rasa/nlu/featurizers/sparse_featurizer/regex_featurizer.py
• rasa/nlu/featurizers/sparse_featurizer/count_vectors_featurizer.py
• rasa/core/featurizers/tracker_featurizers.py:L216
• rasa/core/featurizers/tracker_featurizers.py:L246

Definition of Done

• all listed warnings above are resolved/removed
• @jupyterjazz assigned reviewer

[samsucik]

I think this is really only about removing the warnings from the code in the first 2 files (no docs changes or additional code changes). In the tracker featurisers, we have to remove the entire method that's been deprecated. Still, should be very quick.