docs: how to remove conversion to openai function from index (#21836)

- bind_tools interface is a better alternative.
- openai doesn't use functions but tools in its API now.
- the underlying content appears in some redirects, so will need to
investigate if we can remove.

docs/docs/how_to/index.mdx

@@ -172,7 +172,6 @@ LangChain Tools contain a description of the tool (to pass to the language model
 - [How to: use built-in tools and built-in toolkits](/docs/how_to/tools_builtin)
 - [How to: use a chat model to call tools](/docs/how_to/tool_calling/)
 - [How to: add ad-hoc tool calling capability to LLMs and chat models](/docs/how_to/tools_prompting)
-- [How to: convert LangChain tools to OpenAI functions](/docs/how_to/tools_as_openai_functions)
 - [How to: add a human in the loop to tool usage](/docs/how_to/tools_human)
 - [How to: handle errors when calling tools](/docs/how_to/tools_error)
 - [How to: call tools using multi-modal data](/docs/how_to/tool_calls_multi_modal)