special device removal space accounting fixes

The space in special devices is not included in spa_dspace (or
dsl_pool_adjustedsize(), or the zfs available property). Therefore
there is always at least as much free space in the normal class, as
there is allocated in the special class(es). And therefore, there is
always enough free space to remove a special device.

However, the checks for free space when removing special devices did not
take this into account. This commit corrects that.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #11329