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>