CLEANUP: reg-tests: Remove obsolete no-htx parameter for reg-tests

The legacy HTTP subsystem has been removed. HTX is always enabled.
diff --git a/reg-tests/README b/reg-tests/README
index d05a2a8..6993d97 100644
--- a/reg-tests/README
+++ b/reg-tests/README
@@ -38,7 +38,7 @@
   You must set HAPROXY_PROGRAM environment variable to give the location
   of the HAProxy program to test to vtest:
 
-    $ HAPROXY_PROGRAM=<my haproxy program> vtest [-Dno-htx=] ...
+    $ HAPROXY_PROGRAM=<my haproxy program> vtest ...
 
   The HAProxy VTC files found in HAProxy sources may be run with the reg-tests
   Makefile target. You must set the VTEST_PROGRAM environment variable to