Simplify isUnregisteredDRep

[carlostome]

Description

Small refactor to replace a partially negated predicate in a negated position by an implication.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Any semantic changes to the specifications are documented in CHANGELOG.md
• Code is formatted according to CONTRIBUTING.md
• Self-reviewed the diff

[WhatisRT]

I like this, but it recently came up that using implication as in if can be quite confusing to newcomers (this was in the context of feesOK). Maybe it's not so confusing here, I don't know, but it's probably safer to just match on r anyway.