Remove icon import from collapsible_sidebar

The collapsible sidebar is not utilizing the icon component at all.
During a refactor it was removed in
9fe79221, but due to a usage of the
dynamic component `<component>`, eslint doesn't complain.

app/assets/javascripts/ide/components/panes/collapsible_sidebar.vue

 <script>
 import { mapActions, mapState } from 'vuex';
 import tooltip from '~/vue_shared/directives/tooltip';
-import Icon from '~/vue_shared/components/icon.vue';
 import IdeSidebarNav from '../ide_sidebar_nav.vue';
 
 export default {
@@ -10,7 +9,6 @@ export default {
     tooltip,
   },
   components: {
-    Icon,
     IdeSidebarNav,
   },
   props: {