Fix invalid HTML in investments sidebar

A <div> can't be a direct child of a <ul>. In this case the <ul> tag
didn't make much sense because there isn't a list; just a map.

Note the `z-index` rule which was added in commit 25e1afea48 is ignored
because the map element is statically positioned. I'm not sure why it
was added; can't reproduce the rendering problem both that commit and
commit c5a749212 mention. Maybe back then the element had a `position:
relative` rule or similar somewhere.

app/assets/stylesheets/layout.scss

@@ -977,13 +977,6 @@ footer {
   }
 }
 
-.sidebar-map {
-
-  .map {
-    z-index: 0;
-  }
-}
-
 .sidebar-title {
   border-top: 2px solid $brand;
   display: inline-block;

app/views/budgets/investments/_map.html.erb

@@ -1,5 +1,3 @@
-<ul class="no-bullet sidebar-map">
-  <div class="map">
-    <%= render_map(@map_location, "budgets", false, nil, @investments_map_coordinates) %>
-  </div>
-</ul>
+<div class="map">
+  <%= render_map(@map_location, "budgets", false, nil, @investments_map_coordinates) %>
+</div>