Cleanup deprecation message popover

app/assets/stylesheets/components/user_popover.scss

-.user-popover {
-  padding: $gl-padding-8;
-  line-height: $gl-line-height;
-}

app/assets/stylesheets/pages/milestone.scss

@@ -230,27 +230,6 @@ $status-box-line-height: 26px;
   background-color: $white-light;
 }
-.milestone-deprecation-message {
-  .popover {
-    padding: 0;
-  }
-
-  .popover-body,
-  .popover-content {
-    padding: 0;
-  }
-}
-
-.milestone-popover-body {
-  padding: $gl-padding-8;
-  background-color: $gray-light;
-}
-
-.milestone-popover-footer {
-  padding: $gl-padding-8 $gl-padding;
-  border-top: 1px solid $white-dark;
-}
-
 .milestone-popover-instructions-list {
   padding-left: 2em;

app/views/shared/milestones/_deprecation_message.html.haml

@@ -11,4 +11,5 @@
       %ol.milestone-popover-instructions-list.append-bottom-0
         %li= _('Click any <strong>project name</strong> in the project list below to navigate to the project milestone.').html_safe
         %li= _('Click the <strong>Promote</strong> button in the top right corner to promote it to a group milestone.').html_safe
+    %hr.popover-hr
     .milestone-popover-footer= link_to _('Learn more'), help_page_url('user/project/milestones/index', anchor: 'promoting-project-milestones-to-group-milestones'), class: 'btn btn-link prepend-left-0', target: '_blank'