msg/async/rdma: remove redundant code

msg/async/rdma: remove redundant code

1. remove aync_handler
1). async_handler is never scheduled (which should be scheduled by
    center->dispatch_event_external).
2). async_hander wrapper handle_async_event, which will be called
    in RDMADispatcher::polling.
So, all async_handler related code are removed.

2. fault won't run to_dead, so removed the commented code

Signed-off-by: Roman Penyaev <rpenyaev@suse.de>