FIX: Update logic for displaying admin palette warning (#34121)

Follow-up to
https://github.com/discourse/discourse/commit/7a9cf9356e486cf3eb6a2c2bf6cd6927a6ef59e4,
`_initialUserColorSchemeId` no longer exists — we can simplify to only
showing this warning if the admin isn't using the default theme

<img width="700" alt="image"
src="https://github.com/user-attachments/assets/b32617e6-be1c-4238-847b-0e03f04204d9"
/>

app/assets/javascripts/admin/addon/controllers/admin-customize-colors.js

@@ -64,20 +64,9 @@ export default class AdminCustomizeColorsController extends Controller {
   }
 
   get changedThemePreferences() {
-    // can't check against null, because the default scheme ID is null
-    if (
-      this._initialUserLightColorSchemeId === undefined &&
-      this.defaultTheme
-    ) {
-      this._captureInitialState();
-    }
-
-    const changedColors =
-      this._initialUserColorSchemeId !==
-      this._initialDefaultThemeLightColorSchemeId;
     const changedTheme = this.defaultTheme?.id !== currentThemeId(this.site);
 
-    return changedColors || changedTheme;
+    return changedTheme;
   }
 
   get isUsingDarkMode() {