Revert "Merge pull request #33686 from teskje/heap-metrics"

[teskje]

This reverts the changes made in #33686. They rely on schema evolution support for builtin item migrations, which currently breaks the upgrade checks. Thus, reverting unblocks the release.

See Slack thread.

Motivation

• This PR reverts a known-desirable feature.

Tips for reviewer

As an alternative, we can also merge the forward fix, #33771, if we feel comfortable.

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]

Nightlies fail on this PR because of https://github.com/MaterializeInc/database-issues/issues/9755. If we decide to merge this, we'll have to somehow add exceptions for the PreflightCheckRollback test.