Remove unused gh-infinite-scroll component and infinite-scroll mixin

no issue
- this component and mixin has been replaced by `ember-infinity`

ghost/admin/app/components/gh-infinite-scroll.js

@@ -1,10 +0,0 @@
-import Component from 'ember-component';
-import InfiniteScrollMixin from 'ghost-admin/mixins/infinite-scroll';
-
-export default Component.extend(InfiniteScrollMixin, {
-    actions: {
-        checkScroll() {
-            this._checkScroll();
-        }
-    }
-});

ghost/admin/app/mixins/infinite-scroll.js

@@ -1,42 +0,0 @@
-import Mixin from 'ember-metal/mixin';
-import run from 'ember-runloop';
-
-export default Mixin.create({
-    isLoading: false,
-    triggerPoint: 100,
-
-    /**
-     * Determines if we are past a scroll point where we need to fetch the next page
-     */
-    _checkScroll() {
-        let element = this.get('element');
-        let triggerPoint = this.get('triggerPoint');
-        let isLoading = this.get('isLoading');
-
-        // If we haven't passed our threshold or we are already fetching content, exit
-        if (isLoading || (element.scrollTop + element.clientHeight + triggerPoint <= element.scrollHeight)) {
-            return;
-        }
-
-        this.sendAction('fetch');
-    },
-
-    didInsertElement() {
-        this._super(...arguments);
-
-        let el = this.get('element');
-
-        el.onscroll = run.bind(this, this._checkScroll);
-
-        // run on load, on the offchance that the initial load
-        // did not fill the view.
-        this._checkScroll();
-    },
-
-    willDestroyElement() {
-        this._super(...arguments);
-
-        // turn off the scroll handler
-        this.get('element').onscroll = null;
-    }
-});

ghost/admin/app/templates/components/gh-infinite-scroll.hbs

@@ -1 +0,0 @@
-{{yield (action "checkScroll")}}

ghost/admin/tests/unit/mixins/infinite-scroll-test.js

@@ -1,18 +0,0 @@
-/* jshint expr:true */
-import EmberObject from 'ember-object';
-import InfiniteScrollMixin from 'ghost-admin/mixins/infinite-scroll';
-import {
-    describe,
-    it
-} from 'mocha';
-import {expect} from 'chai';
-
-describe('Unit: Mixin: infinite-scroll', function () {
-    // Replace this with your real tests.
-    it('works', function () {
-        let InfiniteScrollObject = EmberObject.extend(InfiniteScrollMixin);
-        let subject = InfiniteScrollObject.create();
-
-        expect(subject).to.be.ok;
-    });
-});