remove unused "nupdates" tracking from update code

The following discussion from isc-private/bind9!442 should be addressed:

@pspacek

I have another curious question - what is purpose of client->nupdates? Judging by INSISTs I can see it can be either 0 or 1. Is it a cross-callback assert of sorts? Or does it have some other purpose as well?

@each

It dates to commit c426fddf in 2004. It was originally a way of keeping track of whether the client was ready to shut down, alongside similar counters nreads, nrecvs, naccepts and nsends.

We keep track of that differently now, and the counter could go away, but Witold and I overlooked it when we cleaned up its brethren in 2020. You've got a good eye.

IMHO it's out of scope for this MR, but we can address it elsewhere.

@pspacek

Agreed, that one is out of scope. (for this MR)