Remove unused sort-dropdown.html template

src/sidebar/templates/sort-dropdown.html

-<span dropdown keyboard-nav>
-  <button
-    type="button"
-    class="top-bar__btn"
-    dropdown-toggle
-    title="Sort by {{vm.sortKey}}">
-    <i class="h-icon-sort"></i>
-  </button>
-  <div class="dropdown-menu__top-arrow"></div>
-  <ul class="dropdown-menu pull-right" role="menu">
-    <li class="dropdown-menu__row"
-        ng-repeat="key in vm.sortKeysAvailable"
-        ng-click="vm.onChangeSortKey({sortKey: key})"
-        ><span class="dropdown-menu-radio"
-               ng-class="{'is-selected' : vm.sortKey === key}"
-        ></span><a class="dropdown-menu__link" href="">{{key}}</a></li>
-  </ul>
-</span>