MINOR: config: completely remove support for "no option http-use-htx"

This one used to still be supported, emitting a warning about it being
deprecated and the default since 2.1. Let's remove it now.
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index 5b49433..8ac6b30 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -1915,17 +1915,6 @@
 					goto out;
 				}
 
-				/* "[no] option http-use-htx" is deprecated */
-				if (strcmp(cfg_opts2[optnum].name, "http-use-htx") == 0) {
-					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;
-				}
-
 				curproxy->no_options2 &= ~cfg_opts2[optnum].val;
 				curproxy->options2    &= ~cfg_opts2[optnum].val;
 
diff --git a/src/proxy.c b/src/proxy.c
index 014436b..13d2cc8 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -109,7 +109,6 @@
 	{ "http-use-proxy-header",        PR_O2_USE_PXHDR, PR_CAP_FE, 0, PR_MODE_HTTP },
 	{ "http-pretend-keepalive",       PR_O2_FAKE_KA,   PR_CAP_BE, 0, PR_MODE_HTTP },
 	{ "http-no-delay",                PR_O2_NODELAY,   PR_CAP_FE|PR_CAP_BE, 0, PR_MODE_HTTP },
-	{ "http-use-htx",                 0,               PR_CAP_FE|PR_CAP_BE, 0, 0 }, // deprecated
 
 	{"h1-case-adjust-bogus-client",   PR_O2_H1_ADJ_BUGCLI, PR_CAP_FE, 0, PR_MODE_HTTP },
 	{"h1-case-adjust-bogus-server",   PR_O2_H1_ADJ_BUGSRV, PR_CAP_BE, 0, PR_MODE_HTTP },