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_middle/src/ty/closure.rs

@@ -6,7 +6,6 @@ use crate::ty;
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::lang_items::LangItem;
 use rustc_span::Span;
 
 use super::{Ty, TyCtxt};
@@ -113,14 +112,6 @@ impl<'tcx> ClosureKind {
     // This is the initial value used when doing upvar inference.
     pub const LATTICE_BOTTOM: ClosureKind = ClosureKind::Fn;
 
-    pub fn trait_did(&self, tcx: TyCtxt<'tcx>) -> DefId {
-        match *self {
-            ClosureKind::Fn => tcx.require_lang_item(LangItem::Fn, None),
-            ClosureKind::FnMut => tcx.require_lang_item(LangItem::FnMut, None),
-            ClosureKind::FnOnce => tcx.require_lang_item(LangItem::FnOnce, None),
-        }
-    }
-
     /// Returns `true` if a type that impls this closure kind
     /// must also implement `other`.
     pub fn extends(self, other: ty::ClosureKind) -> bool {
@@ -377,12 +368,4 @@ impl BorrowKind {
             UniqueImmBorrow => hir::Mutability::Mut,
         }
     }
-
-    pub fn to_user_str(&self) -> &'static str {
-        match *self {
-            MutBorrow => "mutable",
-            ImmBorrow => "immutable",
-            UniqueImmBorrow => "uniquely immutable",
-        }
-    }
 }