MINOR: connection: Remove the multiplexer protocol PROTO_MODE_HTX

Since the legacy HTTP mode is disabled and no multiplexer relies on it anymore,
there is no reason to have 2 multiplexer protocols for the HTTP. So the protocol
PROTO_MODE_HTX was removed and all HTTP multiplexers use now PROTO_MODE_HTTP.
diff --git a/include/types/connection.h b/include/types/connection.h
index cc2fb17..6609f8b 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -471,8 +471,7 @@
 	PROTO_MODE_NONE = 0,
 	PROTO_MODE_TCP  = 1 << 0, // must not be changed!
 	PROTO_MODE_HTTP = 1 << 1, // must not be changed!
-	PROTO_MODE_HTX  = 1 << 2, // must not be changed!
-	PROTO_MODE_ANY  = PROTO_MODE_TCP | PROTO_MODE_HTTP, // note: HTX is experimental and must not appear here
+	PROTO_MODE_ANY  = PROTO_MODE_TCP | PROTO_MODE_HTTP,
 };
 
 enum proto_proxy_side {