Remove Imagen model from the list of models

The [Imagen models](https://ai.google.dev/gemini-api/docs/deprecations#imagen-models) have been deprecated and will be shutdown on August 17th. It's about time we remove it from this solution :)

Search the repository for references to the Imagen models and identify the model list used by the solution. Remove the deprecated entry and any directly related references, then run the project’s available checks to confirm the remaining model list still works.

Written by the indexing model from the issue text.