workflow delete should (be able to) delete completed workflows

[dandavison]

temporal workflow delete -w $my-completed-workflow-id fails with

Error: unable to delete workflow: operation GetCurrentExecution encountered not found

but a user will expect it to delete running and not-running workflows alike.

The two cases are handled differently by the server deleteworkflow handler:

https://github.com/temporalio/temporal/blob/b53dafa4789981e3dfe6bf5767f7d641b2e690c5/service/history/api/deleteworkflow/api.go#L65-L69