fix(scripts): release an alias twin's local-part netid when it folds

[quntao-z]

Follow-up to #4990, refs #4988.

Applying #4990 on Development folded the 3 alias-twin researchers, and accounts:merge-local-part-netid-twins then refused all 3 account pairs as researcher-identifier-holds-local-part: each archived twin still held the email local part as identifiers.netid.

• The fold now unsets an alias twin's identifiers.netid along with accountId.
  The netid resolver (defaultResolveResearcherIdByNetid) does not skip archived researchers, so a twin still holding the alias keeps answering for it.
• The attribute union no longer offers an alias twin's local-part netid to the real researcher, which would otherwise fill a real researcher that has no netid with the local part.
  Measured on Development: no real researcher received it in the fix(scrapers): resolve an alias-keyed user to the real account and fold alias-twin researchers #4990 apply.
• The integration test covers both, and the second case fails against fix(scrapers): resolve an alias-keyed user to the real account and fold alias-twin researchers #4990's code.

Data operation

The 3 twins archived by the #4990 apply are already out of the fold's reach, so their local-part identifiers.netid is unset directly, which is the write this change makes for every later fold, before accounts:merge-local-part-netid-twins --apply runs on Development.

[github-actions]

Approved automatically: the author holds the Admin repository role (#4512).