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_middle/src/mir/coverage.rs

@@ -117,17 +117,9 @@ impl CoverageKind {
         }
     }
 
-    pub fn is_counter(&self) -> bool {
-        matches!(self, Self::Counter { .. })
-    }
-
     pub fn is_expression(&self) -> bool {
         matches!(self, Self::Expression { .. })
     }
-
-    pub fn is_unreachable(&self) -> bool {
-        *self == Self::Unreachable
-    }
 }
 
 impl Debug for CoverageKind {