Remove varchar_to_text from MIR

[ggevay]

This is an attempt on https://github.com/MaterializeInc/database-issues/issues/1291 as discussed here, at least for the varchar_to_text case.

Draft for now, because I'd like to run a full CI first.

In the meantime, there is a relevant design doc PR from Michael: #27239, where this PR is the eq-type alternative.

Edit: A problem is that the adapter crate also calls MIR's typ in several places, to get RelationDesc's. We'd need to do something about these before merging the PR. A possible solution would be to get the scalar types from the HIR expression instead. This would have sometimes better or worse nullability than MIR's typ, but it being more stable than MIR's typ would probably be an advantage anyway, regardless of this PR.

Motivation

• This PR partially fixes a recognized bug: https://github.com/MaterializeInc/database-issues/issues/1291

Tips for reviewer

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).
• This PR includes the following user-facing behavior changes: