I believe we know what was causing the issue in ThreadList where an
effect callback was sometimes called with closed-over variables which
did not match the most recent render. See [1] for details. The issue
really needs to be resolved in a more general way than trying to work
around it in individual components.

This commit removes code added for debugging this and also adds a check
for a legitimate way (ie. without anything going wrong inside Preact)
that the problem could occur, although it should never happen in the
actual app as we never render `ThreadList` in a container that is
disconnected from the document.

[1] https://github.com/hypothesis/client/pull/3665#issuecomment-895857072