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_mir/src/dataflow/framework/cursor.rs

@@ -64,10 +64,6 @@ where
         }
     }
 
-    pub fn body(&self) -> &'mir mir::Body<'tcx> {
-        self.body
-    }
-
     /// Returns the underlying `Results`.
     pub fn results(&self) -> &Results<'tcx, A> {
         &self.results.borrow()