spl-module-parameters.5: remove spl_kmem_cache_{expire,obj_per_slab_min}

Both were removed in 4fbdb10c7b94439694ad18409662210099e09cb4 ("remove
kmem_cache module parameter KMC_EXPIRE_AGE")

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12157