[Core] Deprecating block manager v1 and make block manager v2 default (#8704)

Removing the block manager v1. This is the initial piece of prefix-caching-centric design. In order to achieve prefix-caching-centric design, we need to simplify the code path so that we only use v2 block manager (which has much higher performance on prefix caching).

examples/offline_inference_mlpspeculator.py

@@ -50,8 +50,6 @@ if __name__ == "__main__":
     llm = LLM(
         model="meta-llama/Llama-2-13b-chat-hf",
         speculative_model="ibm-fms/llama-13b-accelerator",
-        # These are currently required for MLPSpeculator decoding
-        use_v2_block_manager=True,
     )
 
     print("With speculation")