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_lint/src/builtin.rs

@@ -481,7 +481,7 @@ fn has_doc(sess: &Session, attr: &ast::Attribute) -> bool {
         return false;
     }
 
-    if attr.is_value_str() {
+    if attr.value_str().is_some() {
         return true;
     }