[dawn][native] Don't handle availability in timestamp quantization shader

The timestamp quantization shader was used both to quantize, normalize
(to ns values) and zero-out timestamps in the query resolve buffer. The
zeroing-out was redundant because all backends either have guarantees
from the target API that the queries not available are 0, or have special
handling in the backend to zero-out unavailable queries already.

Remove that logic to simplify things a bit. This also allows removing
TimestampParams::first. The dispatch for the quantization is also
updated to be sized for exactly the number of queries that are resolved
instead of running on the whole buffer.

Bug: 499140183
Change-Id: I9b53b595c406d7e3fe05f1405010d64428d133f5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/305055
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>