assistant: Remove /auto (#27608)

This PR removes the `/auto` command.

This was feature-flagged and was never released to the general public.

Release Notes:

- N/A

crates/assistant_slash_commands/Cargo.toml

@@ -30,7 +30,6 @@ http_client.workspace = true
 indexed_docs.workspace = true
 language.workspace = true
 language_model.workspace = true
-log.workspace = true
 project.workspace = true
 prompt_store.workspace = true
 rope.workspace = true