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_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs

@@ -8,12 +8,6 @@ pub struct ImpliedOutlivesBounds<'tcx> {
     pub ty: Ty<'tcx>,
 }
 
-impl<'tcx> ImpliedOutlivesBounds<'tcx> {
-    pub fn new(ty: Ty<'tcx>) -> Self {
-        ImpliedOutlivesBounds { ty }
-    }
-}
-
 impl<'tcx> super::QueryTypeOp<'tcx> for ImpliedOutlivesBounds<'tcx> {
     type QueryResponse = Vec<OutlivesBound<'tcx>>;