[Qt] Fix the Mac build with the current qtbase/dev branch
https://bugs.webkit.org/show_bug.cgi?id=111190

Reviewed by Allan Sandfeld Jensen.

QMAKE_MACOSX_DEPLOYMENT_TARGET now influences
__MAC_OS_X_VERSION_MIN_REQUIRED through -mmacosx-version-min after
changes in qtbase to support iOS.

Having only WebCore defining it to 10.5 would create a mismatch of
ENABLE_THREADING_LIBDISPATCH with WTF to cause undefined symbols
while linking both together at the end.

Fix it by removing the statement which isn't needed anymore.
qtbase/mkspecs/common/mac-minimum-version.conf already defines
10.6 as the minimum Mac version.

* Target.pri:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144464 268f45cc-cd09-0410-ab3c-d52691b4dbfc