compute: don't reset PartitionedComputeState

[teskje]

Resetting the PartitionedComputeState upon seeing a CreateTimely command is not necessary. Both sides of the controller-replica connections throw away their existing partitioned state and create a new instance when the connection fails, implicitly resetting the state.

Motivation

• This PR refactors existing code.

Pulled out from #31710.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[teskje]

TFTR!