LU-13004 net: discard LNET_MD_IOVEC

LU-13004 net: discard LNET_MD_IOVEC

This flag is now never set, so discard it.  lnet_md memory descriptors
are either a single virtual address, or a list of bvec.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I9b2280f801108e5d9fc718c90c83692042991f12
Reviewed-on: https://review.whamcloud.com/36974
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>