Revert "getting ArtifactType from DecisionKey."

This reverts commit fe7a118075668781c9cb1e6af6289b3606f45c29.
Revert "Merging FlowArtifact and DecisionArtifact into mutual artifact."

This reverts commit 849d6c75fbc39912ac63684de5fe7f1a28839fbe.