settings: Use a derive macro for refine (#38451)

When we refactored settings to not pass JSON blobs around, we ended up
needing
to write *a lot* of code that just merged things (like json merge used
to do).

Use a derive macro to prevent typos in this logic.

Release Notes:

- N/A

crates/zlog_settings/src/zlog_settings.rs

@@ -24,17 +24,11 @@ pub struct ZlogSettings {
 }
 
 impl Settings for ZlogSettings {
-    fn from_defaults(content: &settings::SettingsContent, _: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent, _: &mut App) -> Self {
         ZlogSettings {
             scopes: content.log.clone().unwrap(),
         }
     }
 
-    fn refine(&mut self, content: &settings::SettingsContent, _: &mut App) {
-        if let Some(log) = &content.log {
-            self.scopes.extend(log.clone());
-        }
-    }
-
     fn import_from_vscode(_: &settings::VsCodeSettings, _: &mut settings::SettingsContent) {}
 }