apps.core.models.candidate: Removed all of the cached fields.

See #554.
apps.core.models.assignment_group: candidateid-undefined instead of unknown-candidateid.
apps.core.testhelper: Do not run update_identifier.
apps.core.models.candidate: Removed the old tests.
apps.core.models.assignment: Removed update_candidates_identifer().

No longer needed after we removed the cached fields from Candidate.
devilry.coreutils.simplified.metabases: Removed Candidate cached fields.