rip out link guards

rip out link guards

As discussed in
https://github.com/rust-lang/rust/pull/32293#issuecomment-200597130,
adding link guards are a heuristic that is causing undue complications:

- the link guards inject extra public symbols, which is not always OK.
- link guards as implemented could be a non-trivial performance hit,
  because no attempt is made to "de-duplicate" the dependency graph,
  so at worst you have O(N!) calls to the link guard functions.

Nonetheless, link guards are very helpful in detecting errors, so it may
be worth adding them back in some modified form in the future.