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/audio/src/audio_settings.rs

@@ -2,7 +2,6 @@ use std::sync::atomic::{AtomicBool, Ordering};
 
 use gpui::App;
 use settings::{Settings, SettingsStore};
-use util::MergeFrom as _;
 
 #[derive(Clone, Debug)]
 pub struct AudioSettings {
@@ -23,7 +22,7 @@ pub struct AudioSettings {
 
 /// Configuration of audio in Zed
 impl Settings for AudioSettings {
-    fn from_defaults(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
         let audio = &content.audio.as_ref().unwrap();
         AudioSettings {
             control_input_volume: audio.control_input_volume.unwrap(),
@@ -32,17 +31,6 @@ impl Settings for AudioSettings {
         }
     }
 
-    fn refine(&mut self, content: &settings::SettingsContent, _cx: &mut App) {
-        let Some(audio) = content.audio.as_ref() else {
-            return;
-        };
-        self.control_input_volume
-            .merge_from(&audio.control_input_volume);
-        self.control_output_volume
-            .merge_from(&audio.control_output_volume);
-        self.rodio_audio.merge_from(&audio.rodio_audio);
-    }
-
     fn import_from_vscode(
         _vscode: &settings::VsCodeSettings,
         _current: &mut settings::SettingsContent,