Removed preprocessing tests because they are already run in the "keras-preprocessing" repo.

[gabrieldemarmiesse]

Summary

I understood that it was one of the goals of the split of keras into three distinct repos. I don't know if it was one purpose that the tests were left here or if it was just that we forgot to remove them. I think this should be clarified.

Related Issues

#9256

PR Overview

• This PR requires new unit tests [y/n] (make sure tests are included)
• This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• This PR is backwards compatible [y/n]
• This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[fchollet]

These tests are inexpensive and should be kept around in some form since these classes/functions are still part of the API. For reference, we still have applications tests, they're just a lot lighter than before (integration tests). In this case the existing unit tests are already very fast so there's limited upside in replacing them with brand new integration tests.

If you want to remove them, they would have to be replaced with thorough integration tests.

[gabrieldemarmiesse]

Thank you very much for taking the time to clarify it.