Coverity fixes for r300906

lib/libdevdctl/consumer.cc
In Consumer::DisconnectFromDevd, don't close the socket if it's
already closed.

cddl/usr.sbin/zfsd/case_file.cc
lib/libdevdctl/consumer.h
Delete dead code leftover from before devd(8) gained SOCK_SEQPACKET
support

Reported by: Coverity
CID: 1356155, 1356169
Sponsored by: Spectra Logic Corp