Remove serial task queue
SerialTaskQueue is the only Gaudi component to use TBB tasks outside of the scheduler.
This object appears to have been early, speculative development for multithreading Gaudi that is not widely used. Its only client is TBBMessageSvc, which provides thread-safe messaging but is superseded by InertMessageSvc.
Rather than migrate SerialTaskQueue away from TBB tasks (see !1193 (merged)) this MR simply removes it, along with TBBMessageSvc.