RDMA/cxgb4: Remove db_drop_task

Unloading iw_cxgb4 can crash due to the unload code trying to use
db_drop_task, which is uninitialized.  So remove this dead code.
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/cxgb4/device.c

@@ -368,7 +368,6 @@ static void c4iw_rdev_close(struct c4iw_rdev *rdev)
 static void c4iw_remove(struct c4iw_dev *dev)
 {
 	PDBG("%s c4iw_dev %p\n", __func__,  dev);
-	cancel_delayed_work_sync(&dev->db_drop_task);
 	list_del(&dev->entry);
 	if (dev->registered)
 		c4iw_unregister_device(dev);

drivers/infiniband/hw/cxgb4/iw_cxgb4.h

@@ -176,7 +176,6 @@ struct c4iw_dev {
 	struct idr mmidr;
 	spinlock_t lock;
 	struct list_head entry;
-	struct delayed_work db_drop_task;
 	struct dentry *debugfs_root;
 	u8 registered;
 };