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/image_viewer/src/image_viewer_settings.rs

@@ -1,7 +1,6 @@
 use gpui::App;
 pub use settings::ImageFileSizeUnit;
 use settings::Settings;
-use util::MergeFrom;
 
 /// The settings for the image viewer.
 #[derive(Clone, Debug, Default)]
@@ -13,18 +12,9 @@ pub struct ImageViewerSettings {
 }
 
 impl Settings for ImageViewerSettings {
-    fn from_defaults(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
         Self {
             unit: content.image_viewer.clone().unwrap().unit.unwrap(),
         }
     }
-
-    fn refine(&mut self, content: &settings::SettingsContent, _cx: &mut App) {
-        self.unit.merge_from(
-            &content
-                .image_viewer
-                .as_ref()
-                .and_then(|image_viewer| image_viewer.unit),
-        );
-    }
 }