Change markdown header tab anchor links to buttons

What does this MR do?

Changes markdown header tab anchor links to buttons. (see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21988#note_105636972)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

app/assets/javascripts/vue_shared/components/markdown/header.vue

 <script>
   import $ from 'jquery';
-  import { Link } from '@gitlab-org/gitlab-ui';
   import Tooltip from '../../directives/tooltip';
   import ToolbarButton from './toolbar_button.vue';
   import Icon from '../icon.vue';
@@ -10,7 +9,6 @@
       Tooltip,
     },
     components: {
-      'gl-link': Link,
       ToolbarButton,
       Icon,
     },