Remove cx from ThemeSettings (#38836)

Before this change the active theme and icon theme were retrofitted onto
the ThemeSettings.

Now they're in their own new global (GlobalTheme::theme(cx) and
GlobalTheme::icon_theme(cx))

This lets us remove cx from the settings traits, and tidy up a few other
things along the way.

Release Notes:

- N/A

crates/call/src/call_settings.rs

@@ -1,4 +1,3 @@
-use gpui::App;
 use settings::Settings;
 
 #[derive(Debug)]
@@ -8,17 +7,11 @@ pub struct CallSettings {
 }
 
 impl Settings for CallSettings {
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let call = content.calls.clone().unwrap();
         CallSettings {
             mute_on_join: call.mute_on_join.unwrap(),
             share_on_join: call.share_on_join.unwrap(),
         }
     }
-
-    fn import_from_vscode(
-        _vscode: &settings::VsCodeSettings,
-        _current: &mut settings::SettingsContent,
-    ) {
-    }
 }