Remove ConversationalPipeline and Conversation object (#31165)

* Remove ConversationalPipeline and Conversation object, as they have been deprecated for some time and are due for removal

* Update not-doctested.txt

* Fix JA and ZH docs

* Fix JA and ZH docs some more

* Fix JA and ZH docs some more

tests/models/t5/test_modeling_tf_t5.py

@@ -248,7 +248,6 @@ class TFT5ModelTest(TFModelTesterMixin, PipelineTesterMixin, unittest.TestCase):
     all_generative_model_classes = (TFT5ForConditionalGeneration,) if is_tf_available() else ()
     pipeline_model_mapping = (
         {
-            "conversational": TFT5ForConditionalGeneration,
             "feature-extraction": TFT5Model,
             "summarization": TFT5ForConditionalGeneration,
             "text2text-generation": TFT5ForConditionalGeneration,
@@ -314,10 +313,6 @@ class TFT5ModelTest(TFModelTesterMixin, PipelineTesterMixin, unittest.TestCase):
     def test_keras_save_load(self):
         pass
 
-    @unittest.skip("Does not support conversations.")
-    def test_pipeline_conversational(self):
-        pass
-
 
 class TFT5EncoderOnlyModelTester:
     def __init__(
@@ -611,10 +606,6 @@ class TFT5GenerationIntegrationTests(unittest.TestCase):
         expected_output_string = ["Ich liebe es so sehr!", "die Transformatoren sind wirklich erstaunlich"]
         self.assertListEqual(expected_output_string, output_strings)
 
-    @unittest.skip("Does not support conversations.")
-    def test_pipeline_conversational(self):
-        pass
-
 
 @require_tf
 @require_sentencepiece