MINOR: config: Warn only if the option http-use-htx is used with "no" prefix

No warning message is emitted anymore if the option is used to enable the
HTX. But it is still diplayed when the "no" prefix is used to disable the HTX
explicitly. So, for existing configs, we display a warning only if there is a
change in the behavior of HAProxy between the 2.1 and the previous versions.
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index cf3cb56..656031b 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -2009,10 +2009,12 @@
 
 				/* "[no] option http-use-htx" is deprecated */
 				if (!strcmp(cfg_opts2[optnum].name, "http-use-htx")) {
-					ha_warning("parsing [%s:%d]: option '%s' is deprecated and ignored."
-						   " The HTX mode is now the only supported mode.\n",
-						   file, linenum, cfg_opts2[optnum].name);
-					err_code |= ERR_WARN;
+					if (kwm ==KWM_NO) {
+						ha_warning("parsing [%s:%d]: option '%s' is deprecated and ignored."
+							   " The HTX mode is now the only supported mode.\n",
+							   file, linenum, cfg_opts2[optnum].name);
+						err_code |= ERR_WARN;
+					}
 					goto out;
 				}