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_driver/src/lib.rs

@@ -146,6 +146,7 @@ impl<'a, 'b> RunCompiler<'a, 'b> {
     pub fn new(at_args: &'a [String], callbacks: &'b mut (dyn Callbacks + Send)) -> Self {
         Self { at_args, callbacks, file_loader: None, emitter: None, make_codegen_backend: None }
     }
+    /// Used by cg_clif.
     pub fn set_make_codegen_backend(
         &mut self,
         make_codegen_backend: Option<
@@ -155,10 +156,13 @@ impl<'a, 'b> RunCompiler<'a, 'b> {
         self.make_codegen_backend = make_codegen_backend;
         self
     }
-    pub fn set_emitter(&mut self, emitter: Option<Box<dyn Write + Send>>) -> &mut Self {
-        self.emitter = emitter;
-        self
+    /// Used by RLS.
+    pub fn set_emitter(&mut self, emitter: Option<Box<dyn Write + Send>>) -> &mut Self
+    {
+           self.emitter = emitter;
+           self
     }
+    /// Used by RLS.
     pub fn set_file_loader(
         &mut self,
         file_loader: Option<Box<dyn FileLoader + Send + Sync>>,