zfs_refcount_remove: explictly ignore returns

Coverity noticed that sometimes we ignore the return, and sometimes we
don't. Its not wrong, and I like consistent style, so here we are.

Reported-by: Coverity (CID-1564584)
Reported-by: Coverity (CID-1564585)
Reported-by: Coverity (CID-1564586)
Reported-by: Coverity (CID-1564587)
Reported-by: Coverity (CID-1564588)

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15647