test: Remove insignificant_test from tst_qtiff

This test is now passing on all CI platforms.

Task-number: QTBUG-28320
Change-Id: I6e0977980cfe788a087646eb751b113f83e8dd12
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: aavit <eirik.aavitsland@digia.com>

tests/auto/tiff/tiff.pro

@@ -6,5 +6,3 @@ CONFIG += testcase
 
 SOURCES += tst_qtiff.cpp
 RESOURCES += $$PWD/../../shared/images/tiff.qrc
-
-CONFIG+=insignificant_test # QTBUG-28320