qt5: remove obsolete Darwin SDK comment

This partially reverts commit 6b77ed4b187da1d42ec4ee727bcdd9d62baa6db3.

pkgs/development/libraries/qt-5/modules/qtbase.nix

···
       throw "Please add a qtPlatformCross entry for ${plat.config}";
 
   # Per https://doc.qt.io/qt-5/macos.html#supported-versions: build SDK = 13.x or 14.x.
-  # SDK 13.x causes weird linking errors on x86_64-darwin, so use 14.x
   darwinVersionInputs = [
     apple-sdk_14
   ];