zvol: generalise zvol_remove_minors_impl() for single zvol case

zvol_remove_minor_impl() and zvol_remove_minors_impl() should be
identical except for how they select zvols to remove, so lets just use
the same function with a flag to indicate if we should include children
and snapshots or not.

Sponsored-by: Klara, Inc.
Sponsored-by: Railway Corporation
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Fedor Uporov <fuporov.vstack@gmail.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17625