afs: remove unused variable 'afs_voltypes'

fs/afs/volume.c:15:26: warning:
 afs_voltypes defined but not used [-Wunused-const-variable=]

It is not used since commit d2ddc776 ("afs: Overhaul
volume and server record caching and fileserver rotation")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David Howells <dhowells@redhat.com>