engine:fix validation issues in start remove replicated bricks

   Fixing the validation issue when user tries to reduce the replica
count by removing one brick from all the replica pairs. Since it is
replicated volume and removing on replicated brick doesn't requrie
data migration

Change-Id: Ib662c2161e96cc1eac1574bea1fc97cdff5feabf
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>