Confluence check: update outdated comment about zero-variables not being removed not affecting other checks

Method org.eclipse.escet.cif.controllercheck.checks.confluence.ConfluenceCheck.createZeroToOldVarsRelations has the following in its JavaDoc:

     * The 'x0' variables are added here, but they can't be removed after this check, since JavaBDD doesn't allow
     * removing variables from the BDD factory after they have been added. They thus remain after this check. However,
     * they are not used in predicates of other checks, so they are simply ignored there. It shouldn't impact the
     * representation of BDDs, nor the performance of the other checks, since these new variables are added 'on top' of
     * the existing ones (closer to the root of BDDs).

This no longer applies, since each check now converts the CIF specification to an internal representation, and these internal representations are not shared among checks anymore.

Addresses #892