MFC r305333: MFV r304156: 7235 remove unused func dsl_dataset_set_blkptr

MFC r305333: MFV r304156: 7235 remove unused func dsl_dataset_set_blkptr

illumos/illumos-gate@bd56f80007857b960e0981ed0797ad8ec844a96b
https://github.com/illumos/illumos-gate/commit/bd56f80007857b960e0981ed0797ad8ec
844a96b

https://www.illumos.org/issues/7235
  The function dsl_dataset_set_blkptr() is unused. We should remove it.

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>

git-svn-id: svn://svn.freebsd.org/base/stable/10@307283 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f