Remove obsolete dependency on QtXml

A while a go, QtSvg was changed to use the QXmlStream* classes
from QtCore. Now, only tests/auto/qsvggenerator is left with a
QtXml dependency.

This patch also reverts commit
b719b93770393bbf20251e1dffc70498061e612c

Change-Id: I67417a27aa66a2db6d1985cad73e5bab806ad7d2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>