Complete the migration of the impersonation switch from SYSTEM to storage config

DRILL-8155 introduced the configuration of the auth mode for a storage plugin in the storage config. The legacy SYSTEM option drill.exec.impersonation was not removed in that change and continues to control impersonation for Hadoop-based plugins. All plugins should use the new authMode property of their storage config to decided with mode of authentication, including impersonation, at which point the drill.exec.impersonation SYSTEM option can be removed.