Print Preview WebUI: Remove unused mixin

Bug: 973674
Change-Id: I235a1a95ebf760d175dc9800e041f0a1fd955036
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336305Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795219}

chrome/browser/resources/print_preview/ui/print_preview_vars_css.html

@@ -24,9 +24,6 @@
   @media (prefers-color-scheme: dark) {
     html {
       --preview-area-background-color: var(--google-grey-refresh-700);
-      --print-preview-disabled-label: {
-        opacity: var(--cr-disabled-opacity);
-      }
       --print-preview-settings-border: var(--cr-separator-line);
       --iron-icon-fill-color: var(--google-grey-refresh-500);
     }