cleanup of obsolete comments and dead code

This removes the App.Renderer.Buffered, which was not improving on the
direct renderer, which has a simpler implementation and was keept for
this regard.

src/app.zig

@@ -32,7 +32,7 @@ const log = std.log.scoped(.app);
 ///   const zterm = @import("zterm");
 ///   const App = zterm.App(
 ///       union(enum) {},
-///       zterm.Renderer.Plain,
+///       zterm.Renderer.Direct,
 ///       true,
 ///   );
 ///   // later on use