gluster:fix remove brick validator

 Substitute volume replicaCount when replicaCount is not passed
in RemoveBrickParameter.

 When user tries to remove a sub volume from DISTRIBUTE_REPLICATE,
he don't need to pass the replicaCount as he is not reducing the
replica count.

Change-Id: Id845f27f01c022c1fdc3240fcf3a7b5a95563bb4
Bug-Url: https://bugzilla.redhat.com/1034346
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>