How do I remove an expired subscription from my account?

afaik, there's no way to remove it (unless you contact support and specifically ask it to be removed)
and there's no reason to, since it won't affect your subscription in any way
if i were you, i'd keep it there
who knows, it might help in in the future
in tracking your serial numbers and purchases
it's always good to have a reference

Thank you for using our product continually! For your question, here're two scenarios that might match what you want to accomplish:
Scenario 1 - If you want to disable an old serial number on an existing device and enable a new serial number, you can refer to the support page Activating Trend Micro Security using a new serial number
                  Note: Remaining period of old license will be merged to the new one automatically.
Scenario 2 - If you want to delete old serial number in My Account Portal, it is not possible to remove and also not necessary to do so.

Please let us know if either scenario is what you're looking for. If not, please let us know.

By the way, we would like to thank you for being our premium support service subscriber, and please don't forget to utilize 24x7 premium hotline to ask questions or report issues.