rustdoc can't handle re-exported classes

[catamorphism]

rustdoc seems to not be able to handle re-exported classes: the loop in reexport_pass::find_reexport_impl_docs was dying with a non-exhaustive match when it saw a def_id for a class. I "fixed" it by adding a no-op place, but it should really generate docs for classes.

[catamorphism]

@pcwalton just said he was going to rip out the code from rustdoc that handles re-exported impls anyway, so this will be irrelevant once he does that.

[catamorphism]

Non-issue; the module this was in has been deleted altogether.