LU-2734 tests: Remove XXXDEV overrides in init_facets_vars()

init_facets_vars() assigns outputs of xxxdevname() calls to XXXDEVs.
This is incorrect because XXXDEVs are supposed to contain device names
while xxxdevname() calls may print dataset names for ZFS-based facets.
Actually, XXXDEV should not be changed at all.  This patch removes the
XXXDEV assignments.

Change-Id: Id484390cbd814c6fa919c02e390700f914561434
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/5243
Tested-by: Hudson
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>