MFV r316923: 8026 retire zfs_throttle_delay and zfs_throttle_resolution

illumos/illumos-gate@6b036259815954b7ad86d651af18efba672cb7a9
https://github.com/illumos/illumos-gate/commit/6b036259815954b7ad86d651af18efba672cb7a9

https://www.illumos.org/issues/8026

zfs_throttle_delay and zfs_throttle_resolution became disused since the new
write throttling mechanism was introduced.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Andriy Gapon <avg@FreeBSD.org>
MFC after: 1 week