3.0 Deprecation Warnings: Remove Policy deprecations

[ancalita]

Background
These warnings were found in:

• rasa.core.policies.fallback.py:L66

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

• rasa.core.policies.form_policy.py:L51

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

• rasa.core.policies.mapping_policy.py:L49

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

• rasa.core.policies.memoization.py:L87

"f""Please configure the max history in your configuration file, ""
f""currently 'max_history' is set to old default value of ""
f""'{max_history}'. If you want to have infinite max history ""
f""set it to 'None' explicitly. We will change the default value of ""
 f""'max_history' in the future to 'None'."","

• rasa.core.policies.sklearn_policy.py:L124

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

Definition of Done

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

[kedz]

@ancalita @JEM-Mosig @TyDunn TwoStageFallbackPolicy is listed as deprecated in our docs page, but does not have a deprecation warning in its constructor. Is this an oversight and should we also remove it from the code base along with these other deprecated policies?

[kedz]

Also it turns out TwoStageFallbackPolicy is a subclass of FallbackPolicy so I guess it has to go.

[TyDunn]

Is this an oversight?

I'd guess it was or maybe since it was subclass of FallbackPolicy, someone did not think it was necessary