xprtrdma: Remove unused padding variables

Clean up. Remove fields that should have been removed by
commit b3221d6a ("xprtrdma: Remove logic that constructs
RDMA_MSGP type calls").

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>