Remove duplicate declaration of gid_cache_wq.

Fixes: d4186194 ("IB/core: Add generic function to extract IB speed from netdev")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>