language_model: Remove dependency on `inline_completion_button` (#20930)

This PR removes a dependency on the `inline_completion_button` crate
from the `language_model` crate.

We were taking on this dependency solely to call `initiate_sign_in`,
which can easily be moved to the `copilot` crate.

This allows `language_model` to move up in the crate dependency graph.

Release Notes:

- N/A