LU-6016 tests: do not need start MDS in sanity-lfsck test 20a

This patch removes "start $SINGLEMDS" and "mount_client $MOUNT"
from sanity-lfsck test_20a() because these operations are already
done in lfsck_prep().

Test-Parameters: alwaysuploadlogs envdefinitions=SLOW=yes,ENABLE_QUOTA=yes mdtfilesystemtype=ldiskfs mdsfilesystemtype=ldiskfs ostfilesystemtype=ldiskfs mdtcount=1 testlist=sanity-lfsck
Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I75c77519c306c0f74db3441936935252cdedb774
Reviewed-on: http://review.whamcloud.com/13025
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>