RDMA/rdma_cm: Delete rdma_addr_client

The only thing it does is block module unload while work is posted from
rdma_resolve_ip().

However, this is not the right place to do this. The users of
rdma_resolve_ip() must ensure their own module does not unload until
rdma_resolve_ip() calls the callback, or until rdma_addr_cancel() is
called.

Similarly callers to rdma_addr_find_l2_eth_by_grh() must ensure their
module does not unload while they are calling code.

The only two users are already safe, so there is no need for this.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>