Remove unneeded perform_failure method
What does this MR do and why?
After !133273 (merged)
it's no longer possible to create an import failure record for a GitHub
import that lacks external_identifiers, so we don't need this extra
logic in the worker to add it.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #428063 (closed)