app [nfc]: Trim out a now-unnecessary comment on tooltipTheme

This is now the natural thing to do for tooltipTheme, following
recommendations in the upstream docs (thanks to my PR
flutter/flutter#135879!) and doesn't need an explicit comment
explaining it.

Author: chrisbobbe

lib/widgets/app.dart

@@ -134,9 +134,6 @@ class _ZulipAppState extends State<ZulipApp> with WidgetsBindingObserver {
         seedColor: kZulipBrandColor,
       ),
       scaffoldBackgroundColor: const Color(0xfff6f6f6),
-      // `preferBelow: false` seems like a better default for mobile;
-      // the area below a long-press target seems more likely to be hidden by
-      // a finger or thumb than the area above.
       tooltipTheme: const TooltipThemeData(preferBelow: false),
     );