BUILD: opentracing: Fix OT build due to misuse of var_clear()

A build regression was introduced by the commit f508f7e7c5 ("CLEANUP:
opentracing: added flt_ot_smp_init() function"). During backports stage, I
missed to change var_clear() call.

It is 2.4-specific. Thus there is no upstream ID.
diff --git a/addons/ot/src/vars.c b/addons/ot/src/vars.c
index d307b52..a17189c 100644
--- a/addons/ot/src/vars.c
+++ b/addons/ot/src/vars.c
@@ -403,7 +403,7 @@
 
 			FLT_OT_DBG(2, "- '%s' -> '%.*s'", var_name, (int)var->data.u.str.data, var->data.u.str.area);
 
-			size = var_clear(var, 1);
+			size = var_clear(var);
 			flt_ot_smp_init(s, &smp, opt, 0, NULL);
 			var_accounting_diff(vars, smp.sess, smp.strm, -size);