Avoid unnecessary nullptr assignment

We can just leave the dangling pointer value in the `run` object: even
though we just deleted it, there's no need to reset this value because
it will never be used again.  (And even if we did, we don't check
against nullptr anyway so having a nullptr here doesn't make anything
safter than a dangling pointer).

The assignment (into the variant) uses a small amount of CPU (via
std::variant), so better for performance to just leave it dangling.

oxenmq/worker.cpp

@@ -247,7 +247,6 @@ void OxenMQ::proxy_worker_message(OxenMQ::control_message_array& parts, size_t l
 
             if (clear_job) {
                 delete batch;
-                run.to_run = static_cast<detail::Batch*>(nullptr);
             }
         } else {
             assert(run.cat->active_threads > 0);