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

Cargo.lock

@@ -413,7 +413,6 @@ dependencies = [
  "menu",
  "multi_buffer",
  "parking_lot",
- "paths",
  "pretty_assertions",
  "project",
  "prompt_library",
@@ -423,7 +422,6 @@ dependencies = [
  "rope",
  "schemars",
  "search",
- "semantic_index",
  "serde",
  "serde_json_lenient",
  "settings",
@@ -675,7 +673,6 @@ dependencies = [
  "project",
  "prompt_store",
  "rope",
- "semantic_index",
  "serde",
  "serde_json",
  "settings",