MINOR: connection: remove checks for CO_FL_HANDSHAKE before I/O

MINOR: connection: remove checks for CO_FL_HANDSHAKE before I/O

There are still leftovers from the pre-xprt_handshake era with lots
of places where I/O callbacks refrain from receiving/sending if they
see that a handshake is present. This needlessly duplicates the
subscribe calls as it will automatically be done by the underlying
xprt_handshake code when attempting the operation.

The only reason for still checking CO_FL_HANDSHAKE is when we decide
to instantiate xprt_handshake. This patch removes all other ones.