5409 Remove shareiscsi description and example from zfs(1M)

illumos/illumos-gate@b3cff10cdd26674d8dc66e0b349fd185df709fad
https://github.com/illumos/illumos-gate/commit/b3cff10cdd26674d8dc66e0b349fd185df709fad

https://www.illumos.org/issues/5409

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Gordon Ross <gwr@nexenta.com>