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_errors/src/diagnostic_builder.rs

@@ -257,20 +257,6 @@ impl<'a> DiagnosticBuilder<'a> {
         self
     }
 
-    /// See [`Diagnostic::multipart_suggestions()`].
-    pub fn multipart_suggestions(
-        &mut self,
-        msg: &str,
-        suggestions: Vec<Vec<(Span, String)>>,
-        applicability: Applicability,
-    ) -> &mut Self {
-        if !self.0.allow_suggestions {
-            return self;
-        }
-        self.0.diagnostic.multipart_suggestions(msg, suggestions, applicability);
-        self
-    }
-
     /// See [`Diagnostic::tool_only_multipart_suggestion()`].
     pub fn tool_only_multipart_suggestion(
         &mut self,