zfs_scrub_limit tunable is not used anywhere

As a part of scrub/resilver tuning zfs_scrub_limit fell out of use,
but the definition of the variable remained in place.
Moreover various guides still (misleadingly) mention it as a way
to influence resilver/scrub behavior.
This commit removes its finally.

Signed-off-by: Cyril Plisko <cyril.plisko@mountall.com>