LU-14462 gss: remove HAVE_SETNS from lgss_keyring

LU-14462 gss: remove HAVE_SETNS from lgss_keyring

For the sake of simplification, a previous patch removed the config
check that sets HAVE_SETNS, due to the fact that in kernels 3.10+
function setns() necessarily exists.
In this case, all #ifdef on HAVE_SETNS are erroneous because it is
not set whereas the function is actually available.
So remove all references to HAVE_SETNS in the code.

Lustre-change: https://review.whamcloud.com/41967
Lustre-commit: 9d347ae1d6aa642a86b710452b1978ea303dea09

Fixes: 8e88bbfef5 ("LU-12477 lustre: remove obsolete config checks")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iab0726c3e847a210185cc8c9353a79976acb1381
Reviewed-on: https://review.whamcloud.com/43166
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>