MD User manager: Remove obsolete iron-overlay-backdrop and paper-listbox styles.

These elemens are no longer used by chrome://md-user-manager.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I3e68c45cf193585a81f833b90a73633a9e9efc66
Reviewed-on: https://chromium-review.googlesource.com/1080033Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563096}

chrome/browser/resources/md_user_manager/shared_styles.html

@@ -58,17 +58,6 @@
         -webkit-margin-start: 8px;
       }
 
-      paper-listbox {
-        --paper-listbox: {
-          padding: 0;
-        };
-      }
-
-      iron-overlay-backdrop {
-        --iron-overlay-backdrop-background-color: white;
-        --iron-overlay-backdrop-opacity: .8;
-      }
-
       .product-logo {
         content: -webkit-image-set(
             url(../../../app/theme/default_100_percent/%DISTRIBUTION%/product_logo_name_22.png) 1x,