[tests] remove tests from libraries with deprecated support (flax, tensorflow_text, ...) (#39051)
* rm tf/flax tests
* more flax deletions
* revert fixture change
* reverted test that should not be deleted; rm tf/flax test
* revert
* fix a few add-model-like tests
* fix add-model-like checkpoint source
* a few more
* test_get_model_files_only_pt fix
* fix test_retrieve_info_for_model_with_xxx
* fix test_retrieve_model_classes
* relative paths are the devil
* add todo