Remove assistant crate (#30168)

This PR removes the `assistant` crate, as it is no longer used.

Release Notes:

- N/A

crates/agent/Cargo.toml

@@ -23,6 +23,7 @@ anyhow.workspace = true
 assistant_context_editor.workspace = true
 assistant_settings.workspace = true
 assistant_slash_command.workspace = true
+assistant_slash_commands.workspace = true
 assistant_tool.workspace = true
 async-watch.workspace = true
 buffer_diff.workspace = true
@@ -45,6 +46,7 @@ gpui.workspace = true
 heed.workspace = true
 html_to_markdown.workspace = true
 http_client.workspace = true
+indexed_docs.workspace = true
 itertools.workspace = true
 jsonschema.workspace = true
 language.workspace = true