Do not delete someone else's QSystemLocale when constructing another

The QSystemLocale constructor remembers the most-recently-constructed
instance - a dodgy enough proposition at the best of times - and
shares it with much of the rest of QLocale.  There is a global static
instance, actually of a derived singleton class, to which it is
usually set on program start-up.  However, the constructor deleted the
remembered instance before remembering any new instances; there was no
way this could not lead to bad consequences.  So let's not do that.

Change-Id: Ie8f3d655c9d4f75f6ec00a5861d98d6020ecc633
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qlocale.cpp

@@ -594,7 +594,6 @@ static QLocalePrivate *c_private()
 */
 QSystemLocale::QSystemLocale()
 {
-    delete _systemLocale;
     _systemLocale = this;
 
     if (system_data)