Remove TestingEnableTbEmptyEvent if it is no longer used

The TestingEnableTbEmptyEvent logic uses a lot of code and complicates our token bucket refill logic. If we need to keep it working, we can -- but it would be much simpler to remove it, if it is not in use.

Is anybody still using it? We seem to have added it in 0.2.5.2-alpha, back in 2013.