Revert "[library list] added 'plus' icon to the toggle of the dropdown menus …" (#7401)

This reverts commit 798384a638.

frontend/src/pages/my-libs/my-libs.js

@@ -166,7 +166,6 @@ class MyLibraries extends Component {
               <h3 className="sf-heading m-0">
                 {gettext('My Libraries')}
                 <SingleDropdownToolbar
-                  withPlusIcon={true}
                   opList={[
                     { 'text': gettext('New Library'), 'onClick': this.toggleCreateRepoDialog },
                     { 'text': gettext('Deleted Libraries'), 'onClick': this.toggleDeletedReposDialog }