staging: lustre: obd: Remove dead code in precleanup

There used to be several pre-cleanup phases, but
only OBD_CLEANUP_EXPORTS is actually used.  Thus
remove the whole notion of precleanup phases.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7034
Reviewed-on: http://review.whamcloud.com/16061


Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>