assistant: Remove /project (#27660)

This PR removes the `/project` command.

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

Release Notes:

- N/A

crates/assistant_slash_commands/Cargo.toml

@@ -29,11 +29,9 @@ html_to_markdown.workspace = true
 http_client.workspace = true
 indexed_docs.workspace = true
 language.workspace = true
-language_model.workspace = true
 project.workspace = true
 prompt_store.workspace = true
 rope.workspace = true
-schemars.workspace = true
 semantic_index.workspace = true
 serde.workspace = true
 serde_json.workspace = true