Fix memory leak in QXmlQuery::setQuery.

Fix memory leak in QXmlQuery::setQuery.

There is a circular reference dependency between VariableDeclaration
and the classes EvaluationCache, ExpressionVariableReference and
TemplateParameterReference. By removing the explicitly shared pointer
of VariableDeclaration in these classes the circle is broken and memory
is freed correctly.

Task-number: QT-4106
Integrated-by: David Boddie