LU-20 remove unnecessary DMU export patch

LU-20 remove unnecessary DMU export patch

The patch for kernel symbol export for DMU back-ends was added to
facilitate upgrading servers.  However, the upstream ZFS code will
no longer be working on RHEL5 kernels, so this patch is not needed.

Change-Id: Icf443a366fd51c06cb5d02df27b7e70f476fcb63
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1086
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>