Remove setScrollClassName mixin and usage

no issue
- removes `setScrollClassName` mixin that was previously used to add a `.scrolling` class to scrollable elements that showed a shadow at the top of boxes when not scrolled to the top

core/client/app/components/gh-content-preview-content.js

@@ -1,24 +1,12 @@
 import Ember from 'ember';
-import setScrollClassName from 'ghost/utils/set-scroll-classname';
 
-const {Component, run} = Ember;
+const {Component} = Ember;
 
 export default Component.extend({
     classNames: ['content-preview-content'],
 
     content: null,
 
-    didInsertElement() {
-        this._super(...arguments);
-
-        let el = this.$();
-
-        el.on('scroll', run.bind(el, setScrollClassName, {
-            target: el.closest('.content-preview'),
-            offset: 10
-        }));
-    },
-
     didReceiveAttrs(options) {
         this._super(...arguments);
 
@@ -30,13 +18,5 @@ export default Component.extend({
                 el.closest('.content-preview').scrollTop(0);
             }
         }
-    },
-
-    willDestroyElement() {
-        this._super(...arguments);
-
-        let el = this.$();
-
-        el.off('scroll');
     }
 });

core/client/app/components/gh-editor.js

@@ -1,5 +1,4 @@
 import Ember from 'ember';
-import setScrollClassName from 'ghost/utils/set-scroll-classname';
 
 const {Component, computed, run} = Ember;
 const {equal} = computed;
@@ -54,17 +53,6 @@ export default Component.extend({
         // cache these elements for use in other methods
         this.set('$previewViewPort', $previewViewPort);
         this.set('$previewContent', this.$('.js-rendered-markdown'));
-
-        $previewViewPort.on('scroll', run.bind($previewViewPort, setScrollClassName, {
-            target: this.$('.js-entry-preview'),
-            offset: 10
-        }));
-    },
-
-    willDestroyElement() {
-        this._super(...arguments);
-        // removes scroll handlers from the view
-        this.get('$previewViewPort').off('scroll');
     },
 
     actions: {

core/client/app/components/gh-infinite-scroll-box.js

@@ -1,23 +1,7 @@
 import Ember from 'ember';
 import InfiniteScrollMixin from 'ghost/mixins/infinite-scroll';
-import setScrollClassName from 'ghost/utils/set-scroll-classname';
 
-const {Component, run} = Ember;
+const {Component} = Ember;
 
 export default Component.extend(InfiniteScrollMixin, {
-    didInsertElement() {
-        let el = this.$();
-
-        this._super(...arguments);
-
-        el.on('scroll', run.bind(el, setScrollClassName, {
-            target: el.closest('.content-list'),
-            offset: 10
-        }));
-    },
-
-    willDestroyElement() {
-        this._super(...arguments);
-        this.$().off('scroll');
-    }
 });

core/client/app/mixins/ed-editor-scroll.js

@@ -1,5 +1,4 @@
 import Ember from 'ember';
-import setScrollClassName from 'ghost/utils/set-scroll-classname';
 
 const {Mixin, run} = Ember;
 
@@ -75,10 +74,6 @@ export default Mixin.create({
         $el.on('keypress', run.bind(this, this.adjustScrollPosition));
 
         $el.on('scroll', run.bind(this, this.scrollHandler));
-        $el.on('scroll', run.bind($el, setScrollClassName, {
-            target: Ember.$('.js-entry-markdown'),
-            offset: 10
-        }));
     },
 
     /**