IB/cxgb3: remove cxio_dbg.c

cxio_dbg.c is uncompiled since commit 2b540355 ("RDMA/cxgb3: cleanups")
10 years after, we could remove it.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>