Proto,tx_rollup: remove an unreachable case

Context

I noticed that a match case wasn't covered by test coverage, but on analysis, it turns out that's because it was unreachable. By rearranging the code a bit, we can remove the unreachable case.

While I was in there, I fixed a comment typo.

Manually testing the MR

Should not affect functionality; unit tests pass.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR