Update placeholder-shown.js

Author: Marukome0743

lib/hacks/placeholder-shown.js

@@ -1,29 +1,14 @@
 let Selector = require('../selector')
-let utils = require('../utils')
 
 class PlaceholderShown extends Selector {
-  constructor(name, prefixes, all) {
-    super(name, prefixes, all)
-
-    if (this.prefixes) {
-      this.prefixes = utils.uniq(this.prefixes.map(() => '-ms-'))
-    }
-  }
-  
-  /**
-   * Add old mozilla to possible prefixes
-   */
-  possible() {
-    return super.possible().concat(['-moz- old'])
-  }
-
   /**
    * Return different selectors depend on prefix
    */
   prefixed(prefix) {
     if (prefix === '-ms-') {
       return ':-ms-input-placeholder'
-    } else if (prefix === '-moz- old') {
+    }
+    if (prefix === '-moz-') {
       return ':-moz-placeholder'
     }
     return `:${prefix}placeholder-shown`