Since 2016 this mode was opt-in for a long time with the default being
single-threaded asyncio-based server implementation. We believe nobody
actually used multi-threaded ZEO5 server mode for real at all.

Multi-threaded server mode was deprecated this spring
(https://github.com/zopefoundation/ZEO/pull/190), and later found to
have concurrency bug that lead to data corruption
(https://github.com/zopefoundation/ZEO/issues/209).

Given deprecation status, instead of fixing those bugs let's finally
remove the multi-threaded server mode.