LU-16646 krb: get rid of MEMORY private cache for krb creds

LU-16646 krb: get rid of MEMORY private cache for krb creds

On client side, Kerberos credentials for root are cached in MEMORY,
but this is just in addition to the original credentials cache.
As there is no need to cache credentials twice, get rid of this
MEMORY private cache.

Lustre-change: https://review.whamcloud.com/50341
Lustre-commit: 5731acd4997f422e3e5561d0ed73359537a7b65f

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ifd02ee6cfafc27347b3c31e0dbbaab15190cf883
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50651
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>