Admin API endpoint to delete a reported event (#15116)

* Admin api to delete event report

* lint +  tests

* newsfile

* Apply suggestions from code review

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>

* revert changes - move to WorkerStore

* update unit test

* Note that timestamp is in millseconds

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Co-authored-by: David Robertson <david.m.robertson1@gmail.com>