Do not re-create sub_domaines unique key compte

New installations haven't had this unique key since 2014
(d9e24d9703) and only old
installations passing through 3.1.0~a would have had it
added.

Given that the unique key constraint was never dropped during
an AlternC upgrade, I think it should just be dropped here.

Re-creating is problematic when merged with other recent changes
because the length of both the key and column size are longer
than what MySQL/MariaDB support.

install/upgrades/3.5.0.3.sql

@@ -1,3 +1 @@
-alter table sub_domaines drop index compte;
-alter table sub_domaines add UNIQUE (compte,domaine,sub,type,valeur,web_action);
-
+DROP INDEX IF EXISTS compte on `sub_domaines`;