Fixed linting issue for no-redundant-role

refs https://github.com/ember-template-lint/ember-template-lint/blob/master/docs/rule/no-redundant-role.md

- a newer version of ember-template-lint added this linting rule, and it
  looks like an accurate change to make, so this removes the redundant
  roles

Author: daniellockyer

ghost/admin/app/components/dashboard/onboarding/share-modal.hbs

@@ -8,7 +8,7 @@
         {{#let (get-setting "coverImage") as |imageUrl|}}
             {{#if imageUrl}}
                 <div class="gh-share-card-image">
-                    <img src={{imageUrl}} alt="" role="presentation" />
+                    <img src={{imageUrl}} alt="" />
                 </div>
             {{else}}
                 <div class="gh-share-card-image gh-cover-placeholder">{{svg-jar "picture"}}</div>

ghost/admin/app/components/gh-post-bookmark.hbs

@@ -3,15 +3,15 @@
         {{#let (or @post.featureImage (get-setting "coverImage")) as |imageUrl|}}
             {{#if imageUrl}}
                 <div class="gh-post-bookmark-image">
-                    <img src={{imageUrl}} alt="" role="presentation" />
+                    <img src={{imageUrl}} alt="" />
                 </div>
             {{/if}}
             <div class="gh-post-bookmark-content {{if imageUrl "" "no-image"}}">
                 <div class="gh-post-bookmark-title">{{@post.title}}</div>
                 <div class="gh-post-bookmark-text truncate">{{@post.excerpt}}</div>
                 <div class="gh-post-bookmark-details">
                     {{#if (get-setting "icon")}}
-                        <div class="gh-post-bookmark-site-icon"><img src={{get-setting "icon"}} alt="" role="presentation" /></div>
+                        <div class="gh-post-bookmark-site-icon"><img src={{get-setting "icon"}} alt="" /></div>
                     {{/if}}
                     <div class="gh-post-bookmark-site-title">{{get-setting "title"}}</div>
                     <div class="gh-post-bookmark-authors">{{post-author-names @post}}</div>

ghost/admin/app/templates/mentions.hbs

@@ -88,7 +88,7 @@
                                 </div>
                                 {{#if mention.sourceFeaturedImage}}
                                     <div class="gh-mention-thumbnail">
-                                        <img src="{{mention.sourceFeaturedImage}}" alt="" role="presentation">
+                                        <img src="{{mention.sourceFeaturedImage}}" alt="" >
                                     </div>
                                 {{/if}}
                             </div>
@@ -110,7 +110,7 @@
         {{#if this.mentionsInfinityModel}}
             <div class="gh-mentions-sidebar">
                 <div class="gh-mentions-explainer">
-                    <img src="assets/img/mentions-background.png" alt="" role="presentation"/>
+                    <img src="assets/img/mentions-background.png" alt="" />
                     <h2>Introducing Mentions</h2>
                     <p>Whenever another person or site links to you, you’ll receive a notification in your dashboard to show you who’s talking about your content — and, when you link to someone else’s work, they’ll be notified, too.</p>
                 </div>