Remove redundant admin-top-bar element

We weren't using the `admin-top-bar` class since commit e6c1cf774, and
we can always use the `.admin .top-bar` selector if we need to. And the
`row expanded` classes basically give an element a width of 100%, which
is already the default width for block elements.

app/components/layout/admin_header_component.html.erb

@@ -7,7 +7,6 @@
     <% end %>
   </div>
 
-  <div class="expanded row admin-top-bar">
   <div class="top-bar">
     <h1>
       <%= link_to namespace_path do %>
@@ -22,5 +21,4 @@
       <% end %>
     <% end %>
   </div>
-  </div>
 </header>