interoperate with older flux-core releases

Problem: the ABI changes in flux-core 0.70 to raw payload
accessors actually need not impact this project since all
interthread payloads are NUL terminated strings.

The last parameter of flux_msg_get_payload() is changing from an
int pointer to a size_t pointer.  Just pass a NULL and avoid
the entire issue as we don't need the size.

src/shell/plugins/interthread.c

@@ -120,13 +120,8 @@ static void interthread_recv (flux_reactor_t *r,
         }
         if (it->trace_flag) {
             const char *payload;
-            size_t size;
-            if (flux_msg_get_payload (msg, (const void **)&payload, &size) == 0
-                && size > 0)
-                shell_trace ("pmix server %s %.*s",
-                             topic,
-                             (int)size - 1,
-                             payload);
+            if (flux_msg_get_payload (msg, (const void **)&payload, NULL) == 0)
+                shell_trace ("pmix server %s %s", topic, payload);
         }
         for (i = 0; i < it->handler_count; i++) {
             if (!strcmp (topic, it->handlers[i].topic))