assistant: Remove /search (#27661)

This PR removes the `/search` command.

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

Release Notes:

- N/A

crates/assistant_slash_commands/Cargo.toml

@@ -32,7 +32,6 @@ language.workspace = true
 project.workspace = true
 prompt_store.workspace = true
 rope.workspace = true
-semantic_index.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 smol.workspace = true