release-22.1: sql: remove the artifact of canceling the txn-scoped context

[yuzefovich]

Backport 1/1 commits from #84628.
Backport 1/1 commits from #84851.

/cc @cockroachdb/release

---

This commit removes an old artifact of having a txn-scoped context
cancellation that is performed when finishing the txn. As Andrei points
out, this txn-scoped cancellation is likely a leftover from ancient
times and is no longer needed. In particular, this also fixes the bug of
using the span after it was finished (which would occur with high
vmodule on context.go file).

Fixes: #84890.

Release note: None

Release justification: bug fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[andreimatei]

I wouldn't backport this so soon (or at all); I think changing what ctx gets canceled has potential implications we're not aware of.

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @mgartner)

[yuzefovich]

Yeah, I was going to let this bake for a bit on master, but now I'm even more worried about this backport. WDYT about backporting my original version of the fix where put the finish of the hi-jacked span into the cancel function? That seems much safer, yet it still fixes test flakes like #84634 on 22.1 branch.

[yuzefovich]

I propose we let it bake for a week, and if nothing new comes up, we merge it - we do wanna fix the #84890 flake 😄

[yuzefovich]

@mgartner @andreimatei we haven't seen any fallout on master from this change plus 22.1.5 has just been published, so it seems that now is the perfect time to merge this.

[mgartner]

Reviewed 4 of 4 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @yuzefovich)