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.

clippy_lints/src/functions.rs

@@ -516,7 +516,7 @@ fn check_needless_must_use(
                 );
             },
         );
-    } else if !attr.is_value_str() && is_must_use_ty(cx, return_ty(cx, item_id)) {
+    } else if !attr.value_str().is_some() && is_must_use_ty(cx, return_ty(cx, item_id)) {
         span_lint_and_help(
             cx,
             DOUBLE_MUST_USE,

clippy_lints/src/missing_doc.rs

@@ -95,7 +95,7 @@ impl MissingDoc {
 
         let has_doc = attrs
             .iter()
-            .any(|a| a.is_doc_comment() || a.doc_str().is_some() || a.is_value_str() || Self::has_include(a.meta()));
+            .any(|a| a.is_doc_comment() || a.doc_str().is_some() || a.value_str().is_some() || Self::has_include(a.meta()));
         if !has_doc {
             span_lint(
                 cx,