Remove `copilot` and `show_copilot_suggestions` setting aliases (#13167)

This PR removes the Copilot-specific aliases for the
`inline_completions` and `show_inline_completions` settings.

While these aliases were added to maintain backward-compatibility, the
aliasing behavior here can lead to a confusing experience when both keys
end up in the `settings.json`.

Release Notes:

- Breaking Change: Removed the `copilot` alias for the
`inline_completions` setting. If you have settings under `copilot` they
should get moved to `inline_completions`.
- Breaking Change: Removed the `show_copilot_suggestions` alias for the
`show_inline_completions` setting.