Remove unnecessary z-index

(cherry picked from commit 7c5ad4eb)

aleksis/core/static/style.scss

@@ -87,12 +87,6 @@ header, main, footer {
   width: auto;
 }
 
-@media only screen and (max-width: 993px) {
-  header div.nav-wrapper {
-    z-index: -5;
-  }
-}
-
 
 /********/
 /* MAIN */