MINOR: lua: Use vars_unset_by_name_ifexist()

There is no good reason to register a variable name, just to unset
that value that could not even be set without the variable existing.

This change should be safe, may be backported if desired.
diff --git a/src/hlua.c b/src/hlua.c
index ed987d5..b5bf646 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -3512,7 +3512,7 @@
 
 	/* Unset the variable. */
 	smp_set_owner(&smp, s->be, s->sess, s, 0);
-	vars_unset_by_name(name, len, &smp);
+	vars_unset_by_name_ifexist(name, len, &smp);
 	return 0;
 }
 
@@ -3990,7 +3990,7 @@
 
 	/* Unset the variable. */
 	smp_set_owner(&smp, s->be, s->sess, s, 0);
-	vars_unset_by_name(name, len, &smp);
+	vars_unset_by_name_ifexist(name, len, &smp);
 	return 0;
 }
 
@@ -5074,7 +5074,7 @@
 
 	/* Unset the variable. */
 	smp_set_owner(&smp, htxn->p, htxn->s->sess, htxn->s, htxn->dir & SMP_OPT_DIR);
-	vars_unset_by_name(name, len, &smp);
+	vars_unset_by_name_ifexist(name, len, &smp);
 	return 0;
 }