Removed the unnecessary dependency of the QtHelp module on the QtXml one

Removed the unnecessary dependency of the QtHelp module on the QtXml one

Merge-request: 2669
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
(cherry picked from commit 02f55668dada4a23ab84e4d3bda1df8a4984b3ae)

Conflicts:
	mkspecs/features/help.prf
	tools/assistant/lib/lib.pro

Change-Id: Ic102dccd79106f6b21aa702efdb2add31fefd72a
Reviewed-on: http://codereview.qt-project.org/4604
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>