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_session/src/parse.rs

@@ -140,6 +140,7 @@ pub struct ParseSess {
 }
 
 impl ParseSess {
+    /// Used for testing.
     pub fn new(file_path_mapping: FilePathMapping) -> Self {
         let sm = Lrc::new(SourceMap::new(file_path_mapping));
         let handler = Handler::with_tty_emitter(ColorConfig::Auto, true, None, Some(sm.clone()));