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>