OpenZFS 9690 - metaslab of vdev with no space maps was flushed during removal

OpenZFS 9690 - metaslab of vdev with no space maps was flushed during removal

Authored by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/9690
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/4e75ba6826
Closes #8039