Remove unused top notification count functionality

no issue

- in a very old iteration of the admin design we needed a data attribute containing the number of notifications in order to add extra spacing in the styles. This hasn't been necessary for a long time and there are easier ways to do it now than using observers.

ghost/admin/app/components/gh-alerts.js

@@ -1,6 +1,5 @@
 import Component from '@ember/component';
 import {alias} from '@ember/object/computed';
-import {observer} from '@ember/object';
 import {inject as service} from '@ember/service';
 
 export default Component.extend({
@@ -9,9 +8,5 @@ export default Component.extend({
 
     notifications: service(),
 
-    messages: alias('notifications.alerts'),
-
-    messageCountObserver: observer('messages.[]', function () {
-        this.sendAction('notify', this.get('messages').length);
-    })
+    messages: alias('notifications.alerts')
 });

ghost/admin/app/controllers/application.js

@@ -17,13 +17,5 @@ export default Controller.extend({
 
         return (this.get('currentPath') !== 'error404' || this.get('session.isAuthenticated'))
                 && !this.get('currentPath').match(/(signin|signup|setup|reset)/);
-    }),
-
-    topNotificationCount: 0,
-
-    actions: {
-        topNotificationChange(count) {
-            this.set('topNotificationCount', count);
-        }
-    }
+    })
 });

ghost/admin/app/templates/application.hbs

@@ -1,14 +1,14 @@
 {{#gh-app showSettingsMenu=ui.showSettingsMenu}}
     {{#gh-skip-link anchor=".gh-main"}}Skip to main content{{/gh-skip-link}}
 
-    {{gh-alerts notify="topNotificationChange"}}
+    {{gh-alerts}}
 
     <div class="gh-viewport {{if ui.autoNav 'gh-autonav'}} {{if ui.showSettingsMenu 'settings-menu-expanded'}} {{if ui.showMobileMenu 'mobile-menu-expanded'}}">
         {{#if showNavMenu}}
             {{gh-nav-menu open=ui.autoNavOpen}}
         {{/if}}
 
-        {{#gh-main onMouseEnter=(action "closeAutoNav" target=ui) data-notification-count=topNotificationCount}}
+        {{#gh-main onMouseEnter=(action "closeAutoNav" target=ui)}}
             {{outlet}}
         {{/gh-main}}