remove BartForConditionalGeneration.generate (#6659)

As suggested here: https://github.com/huggingface/transformers/issues/6651#issuecomment-678594233
this removes generic `generate` doc with examples not-relevant to bart.

docs/source/model_doc/bart.rst

@@ -32,7 +32,7 @@ BartForConditionalGeneration
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 .. autoclass:: transformers.BartForConditionalGeneration
-    :members: generate, forward
+    :members: forward
 
 
 BartConfig