MDEV-22744 *SAN: sql/item_xmlfunc.cc:791:43: runtime error: downcast of address ... which does not point to an object of type 'Item_func' note: object is of type 'Item_bool' (on optimized builds)

In Item_nodeset_func_predicate::val_nodeset, args[1] is not necessarily
an Item_func descendant. It can be Item_bool.

Removing a wrong cast. It was not really needed anyway.