Get rid of LLMContext.get_messages_for_persistent_storage().

The reason for its `system_instruction` argument was to support usage with LLMs where you might pass the system instruction as a parameter to the `LLMService` rather than specifying it in the context.

But as I thought about it more I became unconvinced that the `system_instruction` argument was really beneficial:

- If you specified your system instruction in your context in the first place, it'll still be there when you read messages for persistent storage
- If you didn't specify your system instruction in the context and instead passed it in as an `LLMService` parameter, you most likely *don't* want it to be in the context when you read messages for persistent storage
- ...and if you really really do need to inject it at the start of the context, it's quite easy to do anyway

And if we remove the `system_instruction` argument from `get_messages_for_persistent_storage()`, then it's essentially just `get_messages()`.

src/pipecat/processors/aggregators/llm_context.py

@@ -131,21 +131,6 @@ class LLMContext:
             )
         return filtered_messages
 
-    def get_messages_for_persistent_storage(
-        self, system_instruction: Optional[str] = None
-    ) -> List[LLMContextMessage]:
-        """Get messages formatted for persistent storage.
-
-        Args:
-            system_instruction: Optional system instruction to ensure is
-                included as the first message in the returned list, if not
-                already present.
-        """
-        messages = copy.deepcopy(self.get_messages())
-        if system_instruction and (not messages or messages[0].get("role") != "system"):
-            messages.insert(0, {"role": "system", "content": system_instruction})
-        return messages
-
     @property
     def tools(self) -> ToolsSchema | NotGiven:
         """Get the tools list.