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_incremental/src/persist/dirty_clean.rs

@@ -74,16 +74,6 @@ const BASE_STRUCT: &[&str] =
     &[label_strs::generics_of, label_strs::predicates_of, label_strs::type_of];
 
 /// Trait definition `DepNode`s.
-const BASE_TRAIT_DEF: &[&str] = &[
-    label_strs::associated_item_def_ids,
-    label_strs::generics_of,
-    label_strs::object_safety_violations,
-    label_strs::predicates_of,
-    label_strs::specialization_graph_of,
-    label_strs::trait_def,
-    label_strs::trait_impls_of,
-];
-
 /// Extra `DepNode`s for functions and methods.
 const EXTRA_ASSOCIATED: &[&str] = &[label_strs::associated_item];
 
@@ -118,10 +108,6 @@ const LABELS_IMPL: &[&[&str]] = &[BASE_HIR, BASE_IMPL];
 /// Abstract data type (struct, enum, union) `DepNode`s.
 const LABELS_ADT: &[&[&str]] = &[BASE_HIR, BASE_STRUCT];
 
-/// Trait definition `DepNode`s.
-#[allow(dead_code)]
-const LABELS_TRAIT: &[&[&str]] = &[BASE_HIR, BASE_TRAIT_DEF];
-
 // FIXME: Struct/Enum/Unions Fields (there is currently no way to attach these)
 //
 // Fields are kind of separate from their containers, as they can change independently from