Remove unused abd_alloc_scatter_offset_chunkcnt

Remove function that become unused after refactoring in
e2af2acce3436acdb2b35fdc7c9de1a30ea85514.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Libby <rlibby@FreeBSD.org>
Closes #11614