NLL: review uses of `revisions: migrate nll` that are already covered by compare-mode

[pnkfelix]

This is just a work item I want to file. We can clean up our test suite slightly by removing occurrences of revisions that are made redundant by compare-mode.

(Note that there are some cases where we deliberately opt out of compare-mode and then use the revisions system to emulate it. Those can stay, though it would be good to review them too.)

[matthewjasper]

Updated this to reflect the change to migrate mode everywhere.

This is waiting for #60171 to land first.

[marmeladema]

I would be happy to help if someone could write some notes about what needs to be done 🙏

[jackh726]

I'm going to go ahead and close this. As mentioned on zulip, I think it makes sense to actually do the opposite here: explicitly move tests with different NLL output to use revisions. This allows us to incrementally review test output differences and add error annotations to the test files. This lowers to burden of review of the final stabilization PR (which should just remove the revision, non-NLL error annotations, and a bless).

compare-mode=nll is good to ensure we don't miss test differences though