MDL-63094 js: Do not check visibility before hiding backdrop

It's possible for the backdrop to be created during the show, before the
modal is actually fully shown.

This check is unnecessary because the getBackdrop() function always
returns a value and will always succeed.

lib/amd/src/modal.js

@@ -577,10 +577,6 @@ define(['jquery', 'core/templates', 'core/notification', 'core/key_codes',
      * @method hide
      */
     Modal.prototype.hide = function() {
-        if (!this.isVisible()) {
-            return;
-        }
-
         this.getBackdrop().done(function(backdrop) {
             if (!this.countOtherVisibleModals()) {
                 // Hide the backdrop if we're the last open modal.