QDate: don't spend code size covering the impossible case

All other similar functions in QDate just return an empty string
in case none of the switch labels would trigger (which would now
cause a compiler warning, after removing the default case label),
so do that here, too.

Saves 44B in text size.

Change-Id: I80ee4975082706adcd15fe89511d08c67e149324
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qdatetime.cpp

@@ -762,7 +762,7 @@ QString QDate::longDayName(int weekday, MonthNameType type)
     case QDate::StandaloneFormat:
         return QLocale::system().standaloneDayName(weekday, QLocale::LongFormat);
     }
-    return QLocale::system().dayName(weekday, QLocale::LongFormat);
+    return QString();
 }
 #endif //QT_NO_TEXTDATE