Do for replication origins what the previous commit did for replication
slots: restore the original behavior of replication origin drop to raise
an error rather than blocking, because users might be depending on the
original behavior.  Maintain the blocking behavior when invoked
internally from logical replication subscription handling.

Discussion: https://postgr.es/m/20170830133922.tlpo3lgfejm4n2cs@alvherre.pgsql