Delete returnDID policy

example/returnDID/1.0/policy.rego

-package example.returnDID
-
-_ = {
-    "@context": ["https://www.w3.org/ns/did/v1", "https://w3id.org/security/suites/ed25519-2020/v1"],
-    "id": issuer().did,
-    "verificationMethod": keys.getAll()
-}
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