fstests: remove privatens test way from check

fstests: remove privatens test way from check

This patch totally revert:
  ce7f796ad check: remove the deprecation of sessionid
  336784e3d check: disable HAVE_PRIVATENS by default
  949bdf8ea check: deprecate using process sessions to isolate test instances
and partially revert:
  247ab01fa check: run tests in a private pid/mount namespace

So it does:
1. Remove "run_privatens" related things from xfstests/check, due to
   check doesn't need that test way.
2. Keep run_privatens script and related changes for check-parallel.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>