chore(styles): remove empty SCSS selector flagged by linter

Signed-off-by: Rito Rhymes <83614463+ritorhymes@users.noreply.github.com>

Author: ritorhymes

src/css/_navigation.scss

@@ -140,11 +140,6 @@
   }
 }
 
-.site-header nav .sub-menu {
-  a {
-  }
-}
-
 .hamburger-click {
   cursor: pointer;
   height: 45px;