MINOR: doc: Remove -Ds option in man page

Remove -Ds option in man page.

Should be backported in every version since 1.8.
diff --git a/doc/haproxy.1 b/doc/haproxy.1
index 86d17cf..5791352 100644
--- a/doc/haproxy.1
+++ b/doc/haproxy.1
@@ -78,10 +78,6 @@
 Start in daemon mode.
 
 .TP
-\fB\-Ds\fP
-Start in systemd daemon mode, keeping a process in foreground.
-
-.TP
 \fB\-q\fP
 Disable messages on output.