StmtNamespaceContext is retained needlessly in xpath expressions

WhenEffectiveStatementImpl with an expression ends up retaining AntlrYangXPathExpression.Qualifed.

This class retains a reference StmtNamespaceContext, which in turn ends up retaining quite a bit of BuildGlobalContext.

While there are cases when this would be necessary, if an XPathExpression does not contain a YangLiteralExpr there is no way the context could/should be referenced, hence we should be losing it and throw ... IllegalArgumentException or similar – as the YangLiteralExpr is invalid anyway.