Remove unnecessary typedef, found by GCC 4.8

qtconcurrentfilter.h:108:47: warning: typedef 'Iterator' locally defined but not used [-Wunused-local-typedefs]

Qt5 commit: d4c241a98059edaa7a4b15beb1cfd16ed041284e (qtbase)
Task-number: QTBUG-30594
Change-Id: Icc69e41c2d4e11962f0c8434236d4847b46e7d9d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>