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;
     }
 

compiler/rustc_lint/src/context.rs

@@ -177,6 +177,7 @@ impl LintStore {
         self.early_passes.push(Box::new(pass));
     }
 
+    /// Used by clippy.
     pub fn register_pre_expansion_pass(
         &mut self,
         pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync,
@@ -862,6 +863,8 @@ impl<'tcx> LateContext<'tcx> {
     ///     // The given `def_id` is that of an `Option` type
     /// }
     /// ```
+    ///
+    /// Used by clippy, but should be replaced by diagnostic items eventually.
     pub fn match_def_path(&self, def_id: DefId, path: &[Symbol]) -> bool {
         let names = self.get_def_path(def_id);
 

compiler/rustc_lint/src/levels.rs

@@ -564,10 +564,6 @@ impl<'s> LintLevelsBuilder<'s> {
         self.id_to_set.insert(id, self.cur);
     }
 
-    pub fn build(self) -> LintLevelSets {
-        self.sets
-    }
-
     pub fn build_map(self) -> LintLevelMap {
         LintLevelMap { sets: self.sets, id_to_set: self.id_to_set }
     }