This PR removes the `clean_certificate_available_date` management command. Recently, a change was made to sync the end date of a course run with a display behavior of "end" as its certificate available date in the Credentials IDA. This change was made to resolve a long running issue with the Credentials IDA not displaying certificates to learners correctly on learner records when the display behavior is set to "end".
This management command, if run, would clear the certificate available date from the course certificate configurations in the Credentials IDA. This would be a regression based on the new functionality. Other improvements have been made to ensure that updates made in the LMS properly flow to Credentials.
TL;DR -- this management command is no longer needed because of feature changes in edx-platform. Allowing this management command to run would break functionality for learners.
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