Remove unnecessary text shadow

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Former-commit-id: 3fafac3e

app/assets/stylesheets/sections/issues.scss

@@ -60,7 +60,6 @@ input.check_all_issues {
 .btn.close_issue {
   color: #B94A48;
   font-weight: bold;
-  @include shade;
   &:hover {
     color: #B94A48;
   }