Fix filtering multi-select fails to deselect options

due to the behavior change in jQuery 3.0
https://jquery.com/upgrade-guide/3.0/#breaking-change-removeattr-no-longer-sets-properties-to-false
Refs. #3389

Author: mshibuya

app/assets/javascripts/rails_admin/ra.filtering-multiselect.js

@@ -225,7 +225,7 @@
     _deSelect: function(options) {
       var widget = this;
       options.each(function(i, option) {
-        widget.element.find('option[value="' + option.value + '"]').removeAttr("selected");
+        widget.element.find('option[value="' + option.value + '"]').prop('selected', false);
       });
       $(options).appendTo(this.collection).prop('selected', false);
     },

app/assets/javascripts/rails_admin/ra.nested-form-hooks.js

@@ -53,7 +53,7 @@
       add_button.addClass('add_nested_fields').html(add_button.data('add-label'));
     }
     field.find('[required]').each(function() {
-      $(this).removeAttr('required');
+      $(this).prop('required', false);
     });
   });
 }(jQuery));