qt6.qtbase: remove xmlstarlet dependency (#412642)

pkgs/development/libraries/qt-6/modules/qtbase/default.nix

···
   which,
   cmake,
   ninja,
-  xmlstarlet,
   libproxy,
   xorg,
   zstd,
···
     pkg-config
     which
     cmake
-    xmlstarlet
     ninja
   ] ++ lib.optionals stdenv.hostPlatform.isDarwin [ moveBuildTree ];