message: go/testutil: drop unneeded methods
  Commit ("go: Allow building an KmsClent using a credential
  Provider") removed the LoadCredentials and LoadDefaultCredentials
  methods from the KMSClient, so there is no longer a need for dummy
  implementations of them.
diff: @@ -124,17 +124,6 @@ func (d *DummyKMSClient) Supported(keyURI string) bool {
  return keyURI == "dummy"
  }
  -// LoadCredentials loads the credentials in credentialPath. If credentialPath is null, loads the
  -// default credentials.
  -func (d *DummyKMSClient) LoadCredentials(credentialPath string) (interface{}, error) {
  - return d, nil
  -}
  -
  -// LoadDefaultCredentials loads with the default credentials.
  -func (d *DummyKMSClient) LoadDefaultCredentials() (interface{}, error) {
  - return d, nil
  -}
  -
  // GetAEAD gets an Aead backend by keyURI.
  func (d *DummyKMSClient) GetAEAD(keyURI string) (tink.AEAD, error) {
  return &DummyAEAD{}, nil
  