Router constraints are useless

[dkfellows]

We have the module pacman.model.constraints.router_constraints which only defines a single abstract class, AbstractRouterConstraint, that nothing checks for or subclasses as a concrete class. Or even as another abstract class. It's use-less, and so is useless.

Can we delete the module? Defining a constraint using it will just result in that constraint being ignored; our routers don't roll that way.

[Christian-B]

Done in #247