Bug 1441284 - Part 7 - Remove the "_ephemeral" property. r=Gijs

The "ephemeral" attribute was not actually used anymore, because it is redundant with the "viewCacheId" attribute.

MozReview-Commit-ID: 6zxf2DmcEfI

browser/components/customizableui/content/panelUI.js

@@ -410,7 +410,6 @@ const PanelUI = {
       multiView.setAttribute("id", "customizationui-widget-multiview");
       multiView.setAttribute("viewCacheId", "appMenu-viewCache");
       multiView.setAttribute("mainViewId", viewNode.id);
-      multiView.setAttribute("ephemeral", true);
       tempPanel.appendChild(multiView);
       viewNode.classList.add("cui-widget-panelview");