Linux: remove unused cr->next member in struct afs_cred

Linux: remove unused cr->next member in struct afs_cred

This field was once used to track allocations in a credentials pool.
That code was removed a few years ago but the field remained.

Change-Id: Ieac642d7d55c4da766a5a3aa8ff790a514b606d6
Reviewed-on: http://gerrit.openafs.org/753
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Dan Hyde <drh@umich.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>