RDMA/rxe: Remove the unused xmit_errors member

The member variable xmit_errors can be replaced with

 rxe_counter_inc(rxe, RXE_CNT_SEND_ERR)

Link: https://lore.kernel.org/r/20211216054842.1099428-1-yanjun.zhu@linux.dev
Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>