Remove eventTime field from class Update type (#26219)

`eventTime` is a vestigial field that can be cleaned up. It was
originally used as part of the starvation mechanism but it's since been
replaced by a per-lane field on the root.

This is a part of a series of smaller refactors I'm doing to
simplify/speed up the `setState` path, related to the Sync Unification
project that @tyao1 has been working on.