refactor: remove omitProviderInModelName functionality and related localization

- Removed the omitProviderInModelName key from PreferenceKeys and its associated logic throughout the codebase.
- Updated SettingsService and AppSettings to eliminate references to the omitted provider functionality.
- Cleaned up related localization strings in multiple languages to reflect the removal of this feature.
- Enhanced code clarity and maintainability by streamlining settings management.

lib/l10n/app_localizations_de.dart

@@ -835,13 +835,6 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get realtime => 'Echtzeit';
 
-  @override
-  String get hideProviderInModelNames => 'Anbieter in Modellnamen ausblenden';
-
-  @override
-  String get hideProviderInModelNamesDescription =>
-      'Zeige Namen wie \"gpt-4o\" statt \"openai/gpt-4o\".';
-
   @override
   String get transportMode => 'Transportmodus';