7235 remove unused func dsl_dataset_set_blkptr

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

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>