message: Fix demand handling on buffer writers
diff: @@ -221,7 +221,6 @@ public:
  if (buf_ && !sub_) {
  CAF_LOG_DEBUG("add subscription");
  sub_ = std::move(sub);
  - sub_.request(buf_->capacity());
  } else {
  CAF_LOG_DEBUG("already have a subscription or buffer no longer valid");
  sub.cancel();
  