fix(extra): make copilot-chat.lua fallback to the latest model (#4411)

## Description

CopilotChat upgraded to gpt-4o early in August
https://github.com/CopilotC-Nvim/CopilotChat.nvim/commit/2c4d2954f9b56945dc8d053903b16759d08f9f37
but the LazyVim plugin is still running with opts of gpt-4. I thought
about changing model to the latest here, but I feel this means more
maintainability for an option which generally is expected to point to
the latest model, and the upstream opts of CopilotChat seem to keep up
with that already.

## Related Issue(s)
None

## Screenshots
None

## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/plugins/extras/coding/copilot-chat.lua

@@ -23,7 +23,6 @@ return {
       local user = vim.env.USER or "User"
       user = user:sub(1, 1):upper() .. user:sub(2)
       return {
-        model = "gpt-4",
         auto_insert_mode = true,
         show_help = true,
         question_header = "  " .. user .. " ",