Settings WebUI: Remove unused mixins

Change-Id: I222db62bbec8664c0ebc97d4086841007bd5a595
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1690713Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675769}

chrome/browser/resources/settings/internet_page/network_summary_item.html

@@ -21,7 +21,6 @@
 
       #outerBox {
         padding: 0 var(--cr-section-padding);
-        @apply(--network-summary-item-outer-box);
       }
 
       #details {
@@ -33,7 +32,6 @@
       #networkTitleText {
         color: #333;
         font-weight: 500;
-        @apply(--network-summary-item-title);
       }
 
       #networkState {