Fix single table inheritance deprecation warning bug

[zarembas]

This fix prevents the deprecation warning Entities was manually included in a model from being raised when using single table inheritance. The warning was being raised because the DfE::Analytics::Entities module gets included in all the entity model classes.

I noticed we were still getting deprecation warnings after this PR got merged: DFE-Digital/apply-for-teacher-training#8374

DEPRECATION WARNING: DfE::Analytics::Entities was manually included in a model (ApplicationWorkExperience), but it's included automatically since v1.4. You're running v1.10.1. To silence this warning, remove the include from model definitions in app/models.
DEPRECATION WARNING: DfE::Analytics::Entities was manually included in a model (ApplicationVolunteeringExperience), but it's included automatically since v1.4. You're running v1.10.1. To silence this warning, remove the include from model definitions in app/models.
DEPRECATION WARNING: DfE::Analytics::Entities was manually included in a model (TextCondition), but it's included automatically since v1.4. You're running v1.10.1. To silence this warning, remove the include from model definitions in app/models.
DEPRECATION WARNING: DfE::Analytics::Entities was manually included in a model (SkeCondition), but it's included automatically since v1.4. You're running v1.10.1. To silence this warning, remove the include from model definitions in app/models.
DEPRECATION WARNING: DfE::Analytics::Entities was manually included in a model (ReferenceCondition), but it's included automatically since v1.4. You're running v1.10.1. To silence this warning, remove the include from model definitions in app/models.

Looking into the code it seems that once the first include runs, all the following classes in that group already have the model included. I initially thought that it gets included in the parent class, but that doesn't seem to be the case.

> mfes = models_for_entity(entity)

> mfes[0].include?(DfE::Analytics::Entities)
=> false
> mfes[1].include?(DfE::Analytics::Entities)
=> false
> mfes[2].include?(DfE::Analytics::Entities)
=> false

> mfes[0].include(DfE::Analytics::Entities)

> mfes[0].include?(DfE::Analytics::Entities)
=> true
> mfes[1].include?(DfE::Analytics::Entities)
=> true
> mfes[2].include?(DfE::Analytics::Entities)
=> true

> mfes[0].superclass
=> ApplicationRecord(abstract)

> mfes[0].superclass.include?(DfE::Analytics::Entities)
=> false

I also removed the @shown_deprecation_warning instance variable as it wasn't serving any purpose.

Tagging @duncanjbrown @JR-G here as I don't have access to this repo

[duncanjbrown]

Thank you!

[zarembas]

FYI I don't have access to this repo, so I can't merge the PR.