Remove hook_exit()

We currently have 2 "end of process" integration points: hook_exit() and the kernel.terminate event.

But really, we just have one. As of #1860026: Change notice: Remove hook_exit() for cached page requests, and decide between it and the onTerminate event otherwise, the only place hook_exit() gets called is from a kernel.terminate listener. All it does is add another layer of abstraction that we don't need, and add a dependency on the module handler that we don't want.

Let's just put hook_exit() out of its misery and eliminate some redundancy in the process.