Remove list_size struct member from list implementation

Removed the list_size struct member as it was only used in a single
assertion, as mentioned in PR #15478.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: MigeljanImeri <imerimigel@gmail.com>
Closes #15812