CLEANUP: proxy: Remove the flag PR_O2_USE_HTX

This flag is now unused. So we can safely remove it.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index ca24dbf..55afed7 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -152,7 +152,7 @@
 #define PR_O2_SRC_ADDR	0x00100000	/* get the source ip and port for logs */
 
 #define PR_O2_FAKE_KA   0x00200000      /* pretend we do keep-alive with server eventhough we close */
-#define PR_O2_USE_HTX   0x00400000      /* use the HTX representation for the HTTP protocol */
+/* unused : 0x00400000 */
 
 #define PR_O2_EXP_NONE  0x00000000      /* http-check : no expect rule */
 #define PR_O2_EXP_STS   0x00800000      /* http-check expect status */
diff --git a/src/http_htx.c b/src/http_htx.c
index 13c1453..3382811 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -680,9 +680,6 @@
 	int err_code = 0;
 
 	for (px = proxies_list; px; px = px->next) {
-		if (!(px->options2 & PR_O2_USE_HTX))
-			continue;
-
 		for (rc = 0; rc < HTTP_ERR_SIZE; rc++) {
 			if (!b_data(&px->errmsg[rc]))
 				continue;
diff --git a/src/proxy.c b/src/proxy.c
index 2a8f87b..a49ca66 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -895,9 +895,6 @@
 	/* initial uuid is unassigned (-1) */
 	p->uuid = -1;
 
-	/* HTX is the default mode, for HTTP and TCP */
-	p->options2 |= PR_O2_USE_HTX;
-
 	/* Default to only allow L4 retries */
 	p->retry_type = PR_RE_CONN_FAILED;