LU-7034 obd: Remove dead code in precleanup

LU-7034 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>
Change-Id: Id1b0922b5d2637aebd409a612c906fe9e15f00d6
Reviewed-on: http://review.whamcloud.com/16061
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>