Don't avoid to load trait impl .class without inliner.

This follows from the discussion in #2963, remove the logic that avoids to load `$class.class` files when the inliner is off.

An alternative could be never to load `$class.class` files (instead of always load them). @gkossakowski said in #2963 that inlining does not work anyway for methods of traits.
However I think this is less regular, and it would definitely take one step *behind* of fixing that issue.
After all, we do load `.class` files for inner classes, which is also pointless in the absence of inlining. So if we really want to do this, we should also avoid loading all the `.class` files not corresponding to a top-level symbol.

See also:
https://issues.scala-lang.org/browse/SI-4767 Methods defined in traits are not inlined