Remove unused generic CSS classes

This removes the `todo`, `todo-pending` and `todo-done` classes from the
component.

These were not used in the Vue implementation, but only carried over
from the old Haml, where they were both used for styling and test
selectors.

app/assets/javascripts/todos/components/todo_item.vue

@@ -48,11 +48,8 @@ export default {
 
 <template>
   <li
-    class="todo gl-border-t gl-border-b gl-relative -gl-mt-px gl-block gl-px-5 gl-py-3 hover:gl-z-1 hover:gl-cursor-pointer hover:gl-border-blue-200 hover:gl-bg-blue-50"
-    :class="{
-      'todo-pending': isPending,
-      'todo-done gl-border-gray-50 gl-bg-gray-10': fadeTodo,
-    }"
+    class="gl-border-t gl-border-b gl-relative -gl-mt-px gl-block gl-px-5 gl-py-3 hover:gl-z-1 hover:gl-cursor-pointer hover:gl-border-blue-200 hover:gl-bg-blue-50"
+    :class="{ 'gl-border-gray-50 gl-bg-gray-10': fadeTodo }"
   >
     <gl-link
       :href="targetUrl"