MFV r336950: 9290 device removal reduces redundancy of mirrors

Mirrors are supposed to provide redundancy in the face of whole-disk failure
and silent damage (e.g. some data on disk is not right, but ZFS hasn't
detected the whole device as being broken). However, the current device
removal implementation bypasses some of the mirror's redundancy.

illumos/illumos-gate@3a4b1be953ee5601bab748afa07c26ed4996cde6

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed by: Sara Hartse <sara.hartse@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Tim Chase <tim@chase2k.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Matthew Ahrens <mahrens@delphix.com>