This keeps in line with how things are done for the fast path
and *might* even provide a *slight* performance increase.

These optimizations result in >2x speedup in the ee-add-remove
benchmark:

* Don't mutate array.length when removing the last listener for
an event
* Don't bother checking max listeners if listeners isn't an array
* Don't call delete when removing the last event in _events, just
re-assign a new object instead