If you really want to force the plant to grow a certain way, then remove it. Years ago, when I lived in zone 4 and only had a few plants, I would control their growth by removing suckers. These days we live in a warmer zone and I don't worry about it. I let the plants branch as much as they want and we get plenty of tomatoes.

I'd leave it. That's how you get more tomatoes!

Thanks for your input. I wondered if the "sucker" would cause smaller fruit growth on the main plant. But since it's so well developed already, I think I will leave it. Thanks again!

If you read this forum much you will learn that there is no such thing as a "sucker". Whether you leave it on should depend on where you are located. Up here in WI I doubt if there is enough time for a new stalk to produce fruit, but if it is as tall as the main stalk then by all means leave it on.