qt_update_submodules: Fix qtx11extras submodule

qtx11extras doesn't have a master branch anymore, dev and stable were created
and are in use instead.

Change-Id: Ifd877ea7d40956f6a585bf327180032e7138065b
Reviewed-by: David Faure (KDE) <faure@kde.org>

scripts/qt/qt_update_submodules.pl

@@ -87,7 +87,6 @@ Readonly my @PROPERTIES => (
 Readonly my %SUBMODULE_TRACKING_REF => (
     qtrepotools  =>  'refs/heads/master',
     qtqa         =>  'refs/heads/master',
-    qtx11extras  =>  'refs/heads/master',
 );
 
 # Author and committer to be used for commits by this script.