Remove KMC_KMEM and KMC_VMEM

KMC_KMEM and KMC_VMEM are now unused since all SPL-implemented
caches are KMC_KVMEM.

KMC_KMEM: Given the default value of spl_kmem_cache_kmem_limit, we
don't use kmalloc to back the SPL caches, instead we use kvmalloc
(KMC_KVMEM). The flag, module parameter, /proc entries, and associated
code are removed.

KMC_VMEM: This flag is not used, and kvmalloc() is always preferable to
vmalloc(). The flag, /proc entries, and associated code are removed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10673