BUG/MINOR: stream_interface: don't return when the fd is already set

BUG/MINOR: stream_interface: don't return when the fd is already set

Back in the days where polling was made with select() where all FDs
were checked at once, stream_int_chk_snd_conn() used to check whether
the file descriptor it was passed was ready or not, so that it did
not perform the work for nothing.

Right now FDs are checked just before calling the I/O handler so this
test never matches at best, or may return false information at worst.

Since conn_fd_handler() always clears the flags upon exit, it looks
like a missed event cannot happen right now. Still, better remove
this outdated check than wait for it to cause issues.