xprtrdma: Clean up unused RPCRDMA_INLINE_PAD_THRESH macro

Fixes: b3221d6a ('xprtrdma: Remove logic that constructs...')
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>