IB/ipoib: Remove unnecessary struct declaration

struct ipoib_cm_tx is defined at 245th line.  And the definition is
independent on the MACRO.  The declaration here is unnecessary. Remove it.

Link: https://lore.kernel.org/r/20210415092124.27684-1-wanjiabing@vivo.com
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/ulp/ipoib/ipoib.h

@@ -677,8 +677,6 @@ void ipoib_cm_handle_rx_wc(struct net_device *dev, struct ib_wc *wc);
 void ipoib_cm_handle_tx_wc(struct net_device *dev, struct ib_wc *wc);
 #else
 
-struct ipoib_cm_tx;
-
 #define ipoib_max_conn_qp 0
 
 static inline int ipoib_cm_admin_enabled(struct net_device *dev)