DISPATCH-2346: Change PN_WEAKREF usage to PN_VOID

PN_VOID is what should have been used initially as the context type in
this record is not a proton PN_OBJECT class type anyway. Moreover
PN_WEAKREF will soon not be exported as it probably can't be used
correctly outside Proton's own libraries.
diff --git a/src/message.c b/src/message.c
index 69827cb..192a434 100644
--- a/src/message.c
+++ b/src/message.c
@@ -1491,7 +1491,7 @@
         qd_alloc_safe_ptr_t sp = QD_SAFE_PTR_INIT(qdl);
         qd_message_set_q2_unblocked_handler((qd_message_t*) msg, qd_link_q2_restart_receive, sp);
         msg->strip_annotations_in  = qd_connection_strip_annotations_in(qdc);
-        pn_record_def(record, PN_DELIVERY_CTX, PN_WEAKREF);
+        pn_record_def(record, PN_DELIVERY_CTX, PN_VOID);
         pn_record_set(record, PN_DELIVERY_CTX, (void*) msg);
         msg->content->max_message_size = qd_connection_max_message_size(qdc);
         qd_link_set_incoming_msg(qdl, (qd_message_t*) msg);