Revert "Add fix for include_usage completion chunk"

This reverts commit 682da59.

haystack/components/generators/chat/openai.py

@@ -416,16 +416,12 @@ async def _handle_async_stream_response(
         chunk = None
 
         async for chunk in chat_completion:  # pylint: disable=not-an-iterable
-            # choices is an empty array for usage_chunk when include_usage is set to True
-            if chunk.usage is not None:
-                chunk_delta = self._convert_usage_chunk_to_streaming_chunk(chunk)
-
-            else:
-                assert len(chunk.choices) == 1, "Streaming responses should have only one choice."
-                chunk_delta: StreamingChunk = self._convert_chat_completion_chunk_to_streaming_chunk(chunk)
+            assert len(chunk.choices) == 1, "Streaming responses should have only one choice."
+            chunk_delta: StreamingChunk = self._convert_chat_completion_chunk_to_streaming_chunk(chunk)
             chunks.append(chunk_delta)
 
             await callback(chunk_delta)
+
         return [self._convert_streaming_chunks_to_chat_message(chunk, chunks)]
 
     def _check_finish_reason(self, meta: Dict[str, Any]) -> None:
@@ -490,14 +486,12 @@ def _convert_streaming_chunks_to_chat_message(
                     _arguments=call_data["arguments"],
                 )
 
-        finish_reason = (chunks[-2] if chunk.usage else chunks[-1]).meta.get("finish_reason")
-
         meta = {
             "model": chunk.model,
             "index": 0,
-            "finish_reason": finish_reason,
+            "finish_reason": chunk.choices[0].finish_reason,
             "completion_start_time": chunks[0].meta.get("received_at"),  # first chunk received
-            "usage": chunk.usage or {},
+            "usage": {},  # we don't have usage data for streaming responses
         }
 
         return ChatMessage.from_assistant(text=text or None, tool_calls=tool_calls, meta=meta)
@@ -565,18 +559,3 @@ def _convert_chat_completion_chunk_to_streaming_chunk(self, chunk: ChatCompletio
             }
         )
         return chunk_message
-
-    def _convert_usage_chunk_to_streaming_chunk(self, chunk: ChatCompletionChunk) -> StreamingChunk:
-        """
-        Converts the usage chunk from the OpenAI API to a StreamingChunk.
-
-        :param chunk: The usage chunk returned by the OpenAI API.
-
-        :returns:
-            The StreamingChunk.
-        """
-        chunk_message = StreamingChunk(content="")
-        chunk_message.meta.update(
-            {"model": chunk.model, "usage": chunk.usage, "received_at": datetime.now().isoformat()}
-        )
-        return chunk_message