MINOR: stream-int: remove useless checks on CS and conn flags in si_cs_send()

MINOR: stream-int: remove useless checks on CS and conn flags in si_cs_send()

In si_cs_send(), some checks are done the CS flags en the connection flags
before calling snd_buf(). But these checks are useless because they have already
been done earlier in the function. The harder to figure out is the flag
CO_FL_SOCK_WR_SH. So it is now tested with CF_SHUTW at the beginning.