Remove <hypothesis-banner> element on destroy

On `destroy` the PDF integration clean up the `<hypothesis-banner>`
element, if any.

src/annotator/integrations/pdf.js

@@ -138,6 +138,7 @@ export class PDFIntegration {
     this._listeners.removeAll();
     this.pdfViewer.viewer.classList.remove('has-transparent-text-layer');
     this.observer.disconnect();
+    this._warningBanner?.remove();
     this._destroyed = true;
   }
 

src/annotator/integrations/test/pdf-test.js

@@ -104,7 +104,6 @@ describe('annotator/integrations/pdf', () => {
       pdfIntegration?.destroy();
       delete window.PDFViewerApplication;
       outerContainer.remove();
-      document.querySelector('hypothesis-banner')?.remove(); // <hypothesis-banner> elements are created outside the outerContainer
       $imports.$restore();
     });