Revert of Hack to fix settings-checkbox mixin for 1.6.0

chrome/browser/resources/settings/controls/settings_checkbox.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-checkbox/paper-checkbox.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_events/cr_events.html">
 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_pref_behavior.html">
 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_pref_indicator.html">
 <link rel="import" href="/controls/pref_control_behavior.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-checkbox">
   <link rel="import" type="css" href="chrome://resources/cr_elements/shared.css">
   <template>
     <style include="settings-shared">
       :host {
         align-items: center;
         display: flex;
         min-height: var(--settings-row-min-height);
       }
 
       .secondary:not(:empty) {
         @apply(--settings-secondary);
       }
 
-      #nonexistent {
-        /* HACK(michaelpg): Inform Polymer that this mixin can set "display". */
-        --settings-secondary-unchecked: {
-          display: block;
-        };
-      }
-
       paper-checkbox:not([checked]) .secondary {
         @apply(--settings-secondary-unchecked);
       }
 
       cr-policy-pref-indicator {
         -webkit-margin-start: var(--checkbox-spacing);
       }
     </style>
     <cr-events id="events"></cr-events>
 
     <div class="layout horizontal center">
       <paper-checkbox id="checkbox" checked="{{checked}}"
           disabled="[[checkboxDisabled_(disabled, pref)]]">
         <div>[[label]]</div>
         <div class="secondary">[[subLabel]]</div>
       </paper-checkbox>
       <template is="dom-if" if="[[pref.policySource]]">
         <cr-policy-pref-indicator pref="[[pref]]"></cr-policy-pref-indicator>
       </template>
     </div>
   </template>
   <script src="settings_checkbox.js"></script>
 </dom-module>