LU-4688 mdt: remove export_put() from mdt_export_evict()

LU-4688 mdt: remove export_put() from mdt_export_evict()

This export reference dropping is not needed here, export
referenced from ptlrpc level and will be dropped there as
well. It looks like that call to the class_export_put()
was added just similar to other places where the
class_fail_export() is called but in those cases export
reference was taken right in the same function while here
it is taken by external caller and will dropped there.

Test-Parameters: envdefinitions=SLOW=yes alwaysuploadlogs testlist=replay-dual
Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I560271668c10715c7f6caa02bfbf2fccab3eeade
Reviewed-on: http://review.whamcloud.com/13706
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>