msg/async/rdma: remove redundant code

msg/async/rdma: remove redundant code

1. Below three bits are meaningless in pollfd::events field:
   POLLERR, POLLHUP, or POLLNVAL.
2. QueuePair::pd is initialized in the initialize list.
   There's no need to assign same value to it.
3. Remove the never used function Chunk::set_bound
4. Remove the never used function Chunk::set_offset
5. Remove the never used function QueuePair::is_error
6. Remove SimplePolicyMessenger used vars
7. remove socket_fd() interface since it's never used.
   All data write/read is based on ConnectedSocketImpl::fd.
   So, there's no need to expose socket_fd since it's never used.
8. Remove RDMAServerSocketImpl::get_fd which is not used.
   BTW, RDMAServerSocketImpl::fd has the same function as get_fd.

Signed-off-by: Changcheng Liu <changcheng.liu@aliyun.com>