They also aren't necessary for anything important to work, and the only effect they have is making the UI say "External Account" instead of "None" under the "Account" column. In particular, the signatures still record the actual email address.
Stop doing this, remove all the references, and destroy all the rows.
(Long ago, Maniphest may also have done this, but no longer does. Nuance/Gatekeeper use a more modern and more suitable "ExternalObject" thing that I initially started adapting here before realizing that Legalpad doesn't actually care about this data.)