Remove legacy STOMP code

This stops mirroring the limbo queue to ActiveMQ.

TODO:

• Use message class to determine whether to normalize?
• In another commit, rename everything that says "Stomp" in reference

to a queue.

• What's up with our timestamp handling? It turns out we were always

taking current timestamp over anything else. Is this correct?

Bug: T99018
Change-Id: Ic835b1d895a742a75a39d986faec6c171db91613