delete ecr_gc_docker job (#71178)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71178
This should no longer be needed as we now set a lifecycle policy on ECR
and we also don't generate lots of temporary containers anymore.
Test Plan: Imported from OSS
Reviewed By: seemethere
Differential Revision: D33537851
Pulled By: suo
fbshipit-source-id: b97b7525be6f62ec8771dfb6a7ee13b22b78ac5a
