Remove long defunct IO_NEWSOCK

Remove long defunct IO_NEWSOCK

Chainsaw used this together with the notify callback to make the iop
data type usable for sockets it listened on, so that io_select() could
multiplex them along with the sockets used for actual I/O.
io_select() became unused in Empire 2, and finally got removed in
commit 875d72a0, v4.2.13.  That made the IO_NEWSOCK and the notify
callback defunct.  The latter got removed in commit 7d5a6b81, v4.3.1.