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/language_models/src/settings.rs

@@ -36,7 +36,7 @@ pub struct AllLanguageModelSettings {
 impl settings::Settings for AllLanguageModelSettings {
     const PRESERVED_KEYS: Option<&'static [&'static str]> = Some(&["version"]);
 
-    fn from_settings(content: &settings::SettingsContent, _cx: &mut App) -> Self {
+    fn from_settings(content: &settings::SettingsContent) -> Self {
         let language_models = content.language_models.clone().unwrap();
         let anthropic = language_models.anthropic.unwrap();
         let bedrock = language_models.bedrock.unwrap();