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_mir/src/util/patch.rs

@@ -117,10 +117,6 @@ impl<'tcx> MirPatch<'tcx> {
         self.add_statement(loc, StatementKind::Assign(box (place, rv)));
     }
 
-    pub fn make_nop(&mut self, loc: Location) {
-        self.make_nop.push(loc);
-    }
-
     pub fn apply(self, body: &mut Body<'tcx>) {
         debug!("MirPatch: make nops at: {:?}", self.make_nop);
         for loc in self.make_nop {