MEDIUM: connection: remove useless flag CO_FL_DATA_WR_SH

MEDIUM: connection: remove useless flag CO_FL_DATA_WR_SH

After careful inspection, this flag is set at exactly two places :
  - once in the health-check receive callback after receipt of a
    response
  - once in the stream interface's shutw() code where CF_SHUTW is
    always set on chn->flags

The flag was checked in the checks before deciding to send data, but
when it is set, the wake() callback immediately closes the connection
so the CO_FL_SOCK_WR_SH flag is also set.

The flag was also checked in si_conn_send(), but checking the channel's
flag instead is enough and even reveals that one check involving it
could never match.

So it's time to remove this flag and replace its check with a check of
CF_SHUTW in the stream interface. This way each layer is responsible
for its shutdown, this will ease insertion of the mux layer.