Remove default viewlet experiment (#15383)

src/vs/workbench/electron-browser/workbench.ts

@@ -503,6 +503,9 @@ export class Workbench implements IPartService {
 
 		// Sidebar visibility
 		this.sideBarHidden = this.storageService.getBoolean(Workbench.sidebarHiddenSettingKey, StorageScope.WORKSPACE, false);
+		if (!this.contextService.getWorkspace()) {
+			this.sideBarHidden = true; // we hide sidebar in single-file-mode
+		}
 
 		const viewletRegistry = Registry.as<ViewletRegistry>(ViewletExtensions.Viewlets);
 		if (!viewletRegistry.getDefaultViewletId()) {