xprtrdma: Remove rpclen from rpcrdma_marshal_req

Clean up: Remove a variable whose result is no longer used.
Commit 655fec69 ("xprtrdma: Use gathered Send for large inline
messages") should have removed it.

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