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_errors/src/lib.rs

@@ -691,10 +691,6 @@ impl Handler {
         db
     }
 
-    pub fn failure(&self, msg: &str) {
-        self.inner.borrow_mut().failure(msg);
-    }
-
     pub fn fatal(&self, msg: &str) -> FatalError {
         self.inner.borrow_mut().fatal(msg)
     }