zpool: fix redundancy check after vdev removal

The presence of indirect vdevs was confusing get_redundancy(), which
considered a pool with e.g. only mirror top-level vdevs and at least
one indirect vdev (due to the removal of a previous vdev) as already
having a broken redundancy, which is not the case. This lead to the
possibility of compromising the redundancy of a pool by adding
mismatched vdevs without requiring the use of -f, and with no
visible notice or warning.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Stéphane Lesimple <speed47_github@speed47.net>
Closes #13705
Closes #13711