Remove feature flag ci_interpolation_inputs_refactor

What does this MR do and why?

• Related to [Feature flag] Enable `ci_interpolation_inputs_... (#418198 - closed)

Remove feature flag ci_interpolation_inputs_refactor

With this feature flag we swapped the existing Ci::Input::Inputs class with Ci::Interpolation::Inputs class which provides a better framework for defining different input types.

The feature flag has been enabled on production: #418198 (comment 1504756554)

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