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/src/mux_h1.c b/src/mux_h1.c
index 009afb0..099cd22 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -479,7 +479,7 @@
 			if (h1c->wait_event.events)
 				conn->xprt->unsubscribe(conn, conn->xprt_ctx,
 				    h1c->wait_event.events, &h1c->wait_event);
-			if (conn_upgrade_mux_fe(conn, NULL, &h1c->ibuf, ist("h2"), PROTO_MODE_HTX) != -1) {
+			if (conn_upgrade_mux_fe(conn, NULL, &h1c->ibuf, ist("h2"), PROTO_MODE_HTTP) != -1) {
 				/* connection successfully upgraded to H2, this
 				 * mux was already released */
 				return;
@@ -2533,7 +2533,7 @@
 
 /* this mux registers default HTX proto */
 static struct mux_proto_list mux_proto_htx =
-{ .token = IST(""), .mode = PROTO_MODE_HTX, .side = PROTO_SIDE_BOTH, .mux = &mux_h1_ops };
+{ .token = IST(""), .mode = PROTO_MODE_HTTP, .side = PROTO_SIDE_BOTH, .mux = &mux_h1_ops };
 
 INITCALL1(STG_REGISTER, register_mux_proto, &mux_proto_htx);