change mrkl-prompt to python version

[devinyf]

change prompt the same as python version:
Current mrkl-prompt in langchaingo would get some very weird answer when I Don't need to use agent-tools in a agent conversation.
Seems like It's forced llm to use some tool to get the Final Answer

And this is langchain python mrkl-prompt:

PREFIX = """Answer the following questions as best you can. You have access to the following tools:"""
FORMAT_INSTRUCTIONS = """Use the following format:
"""

This have no problem

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[tmc]

LGTM