OpenZFS 8026 - retire zfs_throttle_delay and zfs_throttle_resolution

OpenZFS 8026 - retire zfs_throttle_delay and zfs_throttle_resolution

Authored by: Andriy Gapon <avg@FreeBSD.org>
Approved by: Richard Lowe <richlowe@richlowe.net>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/8026
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/9b33e07
Closes #6014