Remove obsolete "Appearance" settings page (#8697)

The "new" design hasn't been worked on in ages, so we might as well remove the support code for now...

Author: Turbo87

app/components/footer.module.css

@@ -7,10 +7,6 @@
     --footer-link-hover-shadow-color: var(--green900);
 }
 
-[data-theme="new-design"] {
-    --footer-bg-color: var(--grey900);
-}
-
 .footer {
     display: grid;
     justify-items: center;

app/components/settings-page.hbs

@@ -1,9 +1,6 @@
 <div local-class="page" ...attributes>
   <SideMenu data-test-settings-menu as |menu|>
     <menu.Item @link={{link "settings.profile"}}>Profile</menu.Item>
-    {{#if this.design.showToggleButton}}
-      <menu.Item @link={{link "settings.appearance"}}>Appearance</menu.Item>
-    {{/if}}
     <menu.Item @link={{link "settings.email-notifications"}}>Email Notifications</menu.Item>
     <menu.Item @link={{link "settings.tokens"}} data-test-tokens>API Tokens</menu.Item>
   </SideMenu>

app/components/settings-page.js

@@ -3,7 +3,6 @@ import { inject as service } from '@ember/service';
 
 export default class ApplicationController extends Controller {
   @service colorScheme;
-  @service design;
   @service progress;
   @service router;
 

app/controllers/application.js

@@ -31,7 +31,6 @@ Router.map(function () {
     this.route('pending-invites');
   });
   this.route('settings', function () {
-    this.route('appearance');
     this.route('email-notifications');
     this.route('profile');
     this.route('tokens', function () {

app/controllers/settings/appearance.js

@@ -85,11 +85,6 @@
      color-scheme: dark;
 }
 
-[data-theme="new-design"] {
-    --header-bg-color: var(--violet800);
-    --main-bg: white;
-}
-
 * {
     box-sizing: border-box;
 }

app/router.js

@@ -1,7 +1,6 @@
 <HeadLayout />
 
 {{page-title "crates.io: Rust Package Registry" separator=' - ' prepend=true}}
-{{set-theme this.design.theme}}
 {{set-color-scheme this.colorScheme.scheme}}
 
 <ProgressBar/>