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 <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #10608 

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