remove LLMResponseStartFrame and LLMResponseEndFrame

This was added in the past to properly handle interruptions for the
LLMAssistantContextAggregator. But this is not necessary anymore since we can
handle interruptions by just processing the StartInterruptionFrame, so there's
no need for these extra frames.

src/pipecat/frames/frames.py

@@ -282,27 +282,13 @@ class EndFrame(ControlFrame):
 
 @dataclass
 class LLMFullResponseStartFrame(ControlFrame):
-    """Used to indicate the beginning of a full LLM response. Following
-    LLMResponseStartFrame, TextFrame and LLMResponseEndFrame for each sentence
-    until a LLMFullResponseEndFrame."""
+    """Used to indicate the beginning of an LLM response. Following by one or
+    more TextFrame and a final LLMFullResponseEndFrame."""
     pass
 
 
 @dataclass
 class LLMFullResponseEndFrame(ControlFrame):
-    """Indicates the end of a full LLM response."""
-    pass
-
-
-@dataclass
-class LLMResponseStartFrame(ControlFrame):
-    """Used to indicate the beginning of an LLM response. Following TextFrames
-    are part of the LLM response until an LLMResponseEndFrame"""
-    pass
-
-
-@dataclass
-class LLMResponseEndFrame(ControlFrame):
     """Indicates the end of an LLM response."""
     pass