theme: Remove unused staff parameter for listing themes (#20077)

This PR removes the `staff` parameter for listing themes, as it was not
used.

Release Notes:

- N/A

crates/languages/Cargo.toml

@@ -38,7 +38,6 @@ async-compression.workspace = true
 async-tar.workspace = true
 async-trait.workspace = true
 collections.workspace = true
-feature_flags.workspace = true
 futures.workspace = true
 gpui.workspace = true
 http_client.workspace = true