Deprecate IntegrationGenerator

[klyonrad]

Is your ticket related to a problem? Please describe.

There is a IntegrationGenerator class and the code comments mention something about "deprecation for this class before rspec-rails 4". I read the commit message and I don't quite understand if it is technically necessary or not? That's why I am opening this ticket for discussion/explanation 🤔

Maybe if we delete the class (and the shared examples) we could reduce the development complexity about generators a bit.

From a user perspective I would say that having that integration generator options is just a bit confusing.

Additional context

#1378

[JonRowe]

We can't delete it now it's made it into rspec-rails 4.0.0 but we should deprecate it and remove it in rspec-rails 5.0.0