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_span/src/def_id.rs

@@ -160,6 +160,8 @@ impl DefPathHash {
     }
 
     /// Returns the crate-local part of the [DefPathHash].
+    ///
+    /// Used for tests.
     #[inline]
     pub fn local_hash(&self) -> u64 {
         self.0.as_value().1