Revert "build: fix wrong variable reference (#23046)"

This reverts commit 1cdbacc.

Author: mmalerba

src/material-experimental/mdc-menu/menu.scss

@@ -98,10 +98,6 @@ mat-menu {
   @include menu-common.item-submenu-trigger(mdc-list-variables.$side-padding);
 }
 
-.mat-mdc-menu-submenu-icon {
-  @include menu-common.item-submenu-icon(mdc-list-variables.$side-padding);
-}
-
 // Increase specificity because ripple styles are part of the `mat-core` mixin and can
 // potentially overwrite the absolute position of the container.
 .mat-mdc-menu-item .mat-mdc-menu-ripple {