LU-18591 sec: remove support for idmap.conf

LU-18591 sec: remove support for idmap.conf

Patch "LU-16630 sec: improve Kerberos cross-realm trust remapping"
added a warning message that idmap.conf is deprecated.
With this patch, we go one step further and remove support for
idmap.conf. This means l_idmap is also removed.
Cross-realm trust remapping should be carried out via a mechanism
provided by Kerberos, such as auth_to_local or equivalent.

Test-Parameters: trivial
Test-Parameters: kerberos=true testlist=sanity-krb5
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I0ab2f3a12d47f55756094043b0ec2b19d8634de1
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57651
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>