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/registry.rs

@@ -13,10 +13,6 @@ impl Registry {
         Registry { long_descriptions: long_descriptions.iter().copied().collect() }
     }
 
-    /// This will panic if an invalid error code is passed in
-    pub fn find_description(&self, code: &str) -> Option<&'static str> {
-        self.long_descriptions[code]
-    }
     /// Returns `InvalidErrorCode` if the code requested does not exist in the
     /// registry. Otherwise, returns an `Option` where `None` means the error
     /// code is valid but has no extended information.