Remove LegacyElementMixin everywhere

Change-Id: I0d8ba8d09d7b12fcfe8adc27df90ffa8bfd8c0fb
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
index 6f35481..b576896 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
@@ -19,7 +19,6 @@
 import '../../../styles/shared-styles';
 import '../../shared/gr-button/gr-button';
 import {DiffViewMode} from '../../../constants/constants';
-import {LegacyElementMixin} from '@polymer/polymer/lib/legacy/legacy-element-mixin';
 import {PolymerElement} from '@polymer/polymer/polymer-element';
 import {htmlTemplate} from './gr-diff-mode-selector_html';
 import {customElement, property} from '@polymer/decorators';
@@ -29,7 +28,7 @@
 import {fireIronAnnounce} from '../../../utils/event-util';
 
 @customElement('gr-diff-mode-selector')
-export class GrDiffModeSelector extends LegacyElementMixin(PolymerElement) {
+export class GrDiffModeSelector extends PolymerElement {
   static get template() {
     return htmlTemplate;
   }