Revert "Add sentencepiece tokenizer support to llm runner"

[jackzhxng]

This is causing

(tr4) [jackzhxng@devvm12068.nha0 ~/tr4/executorch (main)]$ cmake-out/examples/models/llama/llama_main --model_path qwen3_0_6b_new.pte --tokenizer_path ~/hf/models--Qwen--Qwen3-0.6B/snapshots/a9c98e602b9d36d2a2f7ba1eb0f5f31e4e8e5143/tokenizer.json --prompt="Who is th
e president of the US?"                                                                                                                                                                                                                                                   
I tokenizers:regex.cpp:27] Registering override fallback regex                                                                                                                                                                                                            
I tokenizers:regex.cpp:27] Registering override fallback regex                                                                                                                                                                                                            
Setting up pretokenizer...                                                                                                                                                                                                                                                terminate called after throwing an instance of 'std::runtime_error'                                                                                                                                                                                                       
  what():  Unsupported behavior 'Isolated' for Split PreTokenizer. Only 'MergedWithPrevious' is supported.