RDMA/ipoib: Remove unused declarations

There are some declarations without function definition, which are listed
as below:

1. ipoib_ib_tx_timer_func() has also been removed since
   commit 8966e28d ("IB/ipoib: Use NAPI in UD/TX flows")

2. ipoib_pkey_event() has been removed since
   commit ee1e2c82 ("IPoIB: Refresh paths instead of flushing them on
   SM change events")

3. ipoib_mcast_dev_down() has been removed since
   commit 988bd503 ("IPoIB: Fix memory leak of multicast group
   structures")

4. ipoib_pkey_open() has been removed since
   commit dd57c930 ("IB/ipoib: Avoid multicast join attempts with
   invalid P_key")

Remove these unused declarations.

Link: https://patch.msgid.link/r/20240818055702.79547-3-zhangzekun11@huawei.com


Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>