Fix #1214: Static methods in the IR.

Trait impls have been eliminated, in favor of supporting
static methods in any kind of class.

Trait impls are encoded exactly in the same way as on the JVM,
i.e., a class directly extending Object, with static methods.
This is currently the only use of static methods in the IR, but
in theory it can support static methods anywhere.
How about removing it? It is used only once anyway in the logic of `ScalaJSOptimizer`. That piece is anyway aware of what kind of objects have what kind of information...