Remove unused 'required' input for <markdown> component

The editor is only used from the <annotation> component which never
sets this input.

h/static/scripts/directive/markdown.js

@@ -251,7 +251,6 @@ module.exports = function($filter, $sanitize) {
       readOnly: '<',
       text: '<?',
       onEditText: '&',
-      required: '@',
     },
     template: require('../../../templates/client/markdown.html'),
   };

h/templates/client/markdown.html

@@ -15,8 +15,7 @@
 </div>
 <textarea class="form-input form-textarea js-markdown-input"
           ng-show="showEditor()"
-          ng-click="$event.stopPropagation()"
-          ng-required="required"></textarea>
+          ng-click="$event.stopPropagation()"></textarea>
 <div class="styled-text js-markdown-preview"
      ng-class="preview && 'markdown-preview'"
      ng-dblclick="togglePreview()"