sys/vfs/smbfs: Strip out vm_zone code from smbfs_vfsops.c

zone(9) has been depricated where the objcache(9) slab allocator
should be used in place. Here smbfs had already been converted,
however the previous dead implementation of vm_zone was left behind
so tear it out.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>