Delete `twisted.python.threadable.isInIOThread`

[exarkun]

After #5154 is resolved, and the requisite ~12 month period has elapsed, we should get rid of isInIOThread. This will simplify the reactor implementations a bit, since they won't have to reach out to twisted.python.threadable to register the IO thread, and we won't need a gross internal flag to control whether or not they should do this (since only one reactor can be "the" IO thread at a time).