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/settings/src/json_schema.rs

@@ -2,7 +2,6 @@ use schemars::schema::{ArrayValidation, InstanceType, RootSchema, Schema, Schema
 use serde_json::Value;
 
 pub struct SettingsJsonSchemaParams<'a> {
-    pub staff_mode: bool,
     pub language_names: &'a [String],
     pub font_names: &'a [String],
 }