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_query_system/src/cache.rs

@@ -3,7 +3,6 @@
 use crate::dep_graph::{DepContext, DepNodeIndex};
 
 use rustc_data_structures::fx::FxHashMap;
-use rustc_data_structures::sync::HashMapExt;
 use rustc_data_structures::sync::Lock;
 
 use std::hash::Hash;
@@ -34,13 +33,6 @@ impl<Key: Eq + Hash, Value: Clone> Cache<Key, Value> {
     pub fn insert(&self, key: Key, dep_node: DepNodeIndex, value: Value) {
         self.hashmap.borrow_mut().insert(key, WithDepNode::new(dep_node, value));
     }
-
-    pub fn insert_same(&self, key: Key, dep_node: DepNodeIndex, value: Value)
-    where
-        Value: Eq,
-    {
-        self.hashmap.borrow_mut().insert_same(key, WithDepNode::new(dep_node, value));
-    }
 }
 
 #[derive(Clone, Eq, PartialEq)]