Delete defunct rescheduling of timers.

In a much earlier time (e25acce12489ea8fa3ed9e09bdc3b7cb5d6d7f8e), transition
delays were implemented by rescheduling the step callback with a new delay;
thus, d3_timer had to support finding an already-registered timer callback and
changing its delay. This was not supposed to be part of the public API when
d3.timer was later exposed, and is no longer needed due to the Great Transition
Rewrite of D3 3.0. So, we can simplify and optimize the code by deleting this
defunct feature.