remove icon on timeline back button

it's cleaner looking this way.. omit needless, uh, ICONS!

app/assets/javascripts/discourse/widgets/topic-timeline.js.es6

@@ -19,7 +19,6 @@ createWidget('timeline-last-read', {
       iconNode('circle', { class: 'progress' }),
       this.attach('button', {
         className: 'btn btn-primary btn-small',
-        icon: 'arrow-left',
         label: 'topic.timeline.back',
         title: 'topic.timeline.back_description',
         action: 'goBack'