Remove (lots of) dead code

Found with https://github.com/est31/warnalyzer.

Dubious changes:
- Is anyone else using rustc_apfloat? I feel weird completely deleting
  x87 support.
- Maybe some of the dead code in rustc_data_structures, in case someone
  wants to use it in the future?
- Don't change rustc_serialize

  I plan to scrap most of the json module in the near future (see
  https://github.com/rust-lang/compiler-team/issues/418) and fixing the
  tests needed more work than I expected.

TODO: check if any of the comments on the deleted code should be kept.

compiler/rustc_query_system/src/dep_graph/debug.rs

@@ -52,6 +52,7 @@ impl EdgeFilter {
         }
     }
 
+    #[cfg(debug_assertions)]
     pub fn test<K: DepKind>(&self, source: &DepNode<K>, target: &DepNode<K>) -> bool {
         self.source.test(source) && self.target.test(target)
     }