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_hir_pretty/src/lib.rs

@@ -221,10 +221,6 @@ pub fn bounds_to_string<'b>(bounds: impl IntoIterator<Item = &'b hir::GenericBou
     to_string(NO_ANN, |s| s.print_bounds("", bounds))
 }
 
-pub fn param_to_string(arg: &hir::Param<'_>) -> String {
-    to_string(NO_ANN, |s| s.print_param(arg))
-}
-
 pub fn ty_to_string(ty: &hir::Ty<'_>) -> String {
     to_string(NO_ANN, |s| s.print_type(ty))
 }
@@ -1701,21 +1697,10 @@ impl<'a> State<'a> {
         }
     }
 
-    pub fn print_usize(&mut self, i: usize) {
-        self.s.word(i.to_string())
-    }
-
     pub fn print_name(&mut self, name: Symbol) {
         self.print_ident(Ident::with_dummy_span(name))
     }
 
-    pub fn print_for_decl(&mut self, loc: &hir::Local<'_>, coll: &hir::Expr<'_>) {
-        self.print_local_decl(loc);
-        self.s.space();
-        self.word_space("in");
-        self.print_expr(coll)
-    }
-
     pub fn print_path(&mut self, path: &hir::Path<'_>, colons_before_params: bool) {
         self.maybe_print_comment(path.span.lo());
 
@@ -2430,24 +2415,6 @@ impl<'a> State<'a> {
         }
     }
 
-    pub fn print_opt_abi_and_extern_if_nondefault(&mut self, opt_abi: Option<Abi>) {
-        match opt_abi {
-            Some(Abi::Rust) => {}
-            Some(abi) => {
-                self.word_nbsp("extern");
-                self.word_nbsp(abi.to_string())
-            }
-            None => {}
-        }
-    }
-
-    pub fn print_extern_opt_abi(&mut self, opt_abi: Option<Abi>) {
-        if let Some(abi) = opt_abi {
-            self.word_nbsp("extern");
-            self.word_nbsp(abi.to_string())
-        }
-    }
-
     pub fn print_fn_header_info(&mut self, header: hir::FnHeader, vis: &hir::Visibility<'_>) {
         self.s.word(visibility_qualified(vis, ""));