MINOR: connection: remove some unneeded checks for CO_FL_SOCK_WR_SH

A few places in health checks and stream-int on the send path were still
checking for this flag. Now we do not and instead we rely on snd_buf()
to report the error if any.

It's worth noting that all 3 real muxes still use CO_FL_SOCK_WR_SH and
CO_FL_ERROR interchangeably at various places to decide to abort and/or
free their data. This should be clarified and fixed so that only
CO_FL_ERROR is used, and this will render the error paths simpler and
more accurate.
diff --git a/src/checks.c b/src/checks.c
index f30f1ae..5f1cec9 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -765,13 +765,6 @@
 		goto out_wakeup;
 	}
 
-	if (conn->flags & CO_FL_SOCK_WR_SH) {
-		/* if the output is closed, we can't do anything */
-		conn->flags |= CO_FL_ERROR;
-		chk_report_conn_err(check, 0, 0);
-		goto out_wakeup;
-	}
-
 	/* here, we know that the connection is established. That's enough for
 	 * a pure TCP check.
 	 */
@@ -3036,12 +3029,6 @@
 				b_reset(&check->bi);
 			}
 
-			if (conn->flags & CO_FL_SOCK_WR_SH) {
-				conn->flags |= CO_FL_ERROR;
-				chk_report_conn_err(check, 0, 0);
-				goto out_end_tcpcheck;
-			}
-
 			if (check->current_step->string_len >= b_size(&check->bo)) {
 				chunk_printf(&trash, "tcp-check send : string too large (%d) for buffer size (%u) at step %d",
 					     check->current_step->string_len, (unsigned int)b_size(&check->bo),
diff --git a/src/stream_interface.c b/src/stream_interface.c
index 14e6627..4e307ed 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -668,7 +668,7 @@
 		return 0;
 
 	/* we might have been called just after an asynchronous shutw */
-	if (conn->flags & CO_FL_SOCK_WR_SH || oc->flags & CF_SHUTW)
+	if (oc->flags & CF_SHUTW)
 		return 1;
 
 	if (oc->pipe && conn->xprt->snd_pipe && conn->mux->snd_pipe) {