Simplify fn calling usage

[enyst]

Give a summary of what the PR does, explaining any non-trivial design decisions

This PR proposes to clean up mock_function_calling var from the agent. The llm knows if it supports it.

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:cdb9248-nikolaik   --name openhands-app-cdb9248   docker.all-hands.dev/all-hands-ai/openhands:cdb9248

[enyst]

This isn't really used in the agent, it's used in llm.py