remove kmem_cache module parameter KMC_EXPIRE_AGE

By default, spl_kmem_cache_expire is KMC_EXPIRE_MEM, meaning that
objects will be removed from kmem cache magazines by
spl_kmem_cache_reap_now().

There is also a module parameter to change this to KMC_EXPIRE_AGE,
which establishes a maximum lifetime for objects to stay in the
magazine. This setting has rarely, if ever, been used, and is not
regularly tested.

This commit removes the code for KMC_EXPIRE_AGE, and associated module
parameters.

Additionally, the unused module parameter
spl_kmem_cache_obj_per_slab_min is removed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10608