tunables: remove legacy FreeBSD aliases

These are old pre-OpenZFS tunable names that have long been
available via either conventional ZFS_MODULE_PARAM tunables or through
kstats. There's no point doubling up anymore, so delete them.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17375