Autotests: Do not set the QT_USE_FAST_CONCATENATION macro in qbs build.

The one test for which we included it does not need it anymore,
and the StringUtils test fails to link on Windows when it is present,
so remove it again.

Change-Id: Iebe810c82842047a323f65722a61940271f921f3
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

tests/auto/autotest.qbs

@@ -15,7 +15,6 @@ CppApplication {
         buildDirectory + '/' + project.ide_library_path + "/..", // OSX
         buildDirectory + '/' + project.ide_plugin_path + "/QtProject"
     ]
-    cpp.defines: base.concat(["QT_USE_FAST_OPERATOR_PLUS"])
 
     // The following would be conceptually right, but does not work currently as some autotests
     // (e.g. extensionsystem) do not work when installed, because they want hardcoded