Fixes a panic that was introduced in #42633. Repro steps:
1. Open the inline assistant and mention a file in the prompt
2. Run the inline assistant
3. Remove the mention and insert a different one
4. 💥

This would happen because the mention set still had a reference to the
old editor, because we create a new one in `PromptEditor::unlink`.

Also removes the unused
`crates/agent_ui/src/context_picker/completion_provider.rs` file, which
was not removed by mistake in the previous PR.

Release Notes:

- N/A