Remove display-options-display-view-mirroring.

In the CSS, this name is used for an ID, but in the JS side
this name is used for a class name, it seems this name is just
unused.

It would be okay to use this as a class name, but this specifies
margin only, and we don't see any claims about margin for mirroring
view. Actually, mirroring displays are centered and sized well,
so nobody notices the margin for the container. Let's just remove
unused code.

BUG=None
R=dbeam@chromium.org

Review URL: https://codereview.chromium.org/967623002

Cr-Commit-Position: refs/heads/master@{#318534}

chrome/browser/resources/options/chromeos/display_options.css

@@ -20,10 +20,6 @@
   width: 100%;
 }
 
-#display-options-displays-view-mirroring {
-  margin: 20px 0 20px 0;
-}
-
 #display-configurations {
   -webkit-padding-end: 0;
   -webkit-padding-start: 15px;

chrome/browser/resources/options/chromeos/display_options.js

@@ -747,8 +747,6 @@ cr.define('options', function() {
       var totalHeight = height + numDisplays * MIRRORING_OFFSET_PIXELS;
 
       this.displaysView_.style.height = totalHeight + 'px';
-      this.displaysView_.classList.add(
-          'display-options-displays-view-mirroring');
 
       // The displays should be centered.
       var offsetX =