3.0 Deprecation Warnings: Remove deprecated featurizers

[ancalita]

Background
These warnings were found in:

• rasa.core.featurizers.single_state_featurizer.py:L382, L410

"f""'{self.__class__.__name__}' is deprecated and ""
f""will be removed in Rasa Open Source 3.0.0. ""
f""It is recommended to use the '{SingleStateFeaturizer.__name__}' instead."""

• rasa.nlu.utils.hugging_face.hf_transformers.py:L66

"f""'{self.__class__.__name__}' is deprecated and ""
f""will be removed in the future. ""
 f""It is recommended to use the '{LanguageModelFeaturizer.__name__}' instead."""

Definition of Done:

• resolve warnings in 3.0 version
• Removed the deprecated featurizers from the codebase.
• Added a changelog entry for 3.0 saying we've removed the previously deprecated featurizers
• Removed the references in the docs

[kedz]

Since tokenization and featurization tests are tightly coupled, I'm removing rasa/nlu/utils/hugging_face/hf_transformers.py as part of #9120 .