Remove SortedMap::iter_mut() since that allows to break the element sorting...

Remove SortedMap::iter_mut() since that allows to break the element sorting order which lookup relies on.

src/librustc_data_structures/sorted_map.rs

@@ -141,14 +141,6 @@ pub fn range<R>(&self, range: R) -> &[(K, V)]
         (&self.data[start .. end])
     }
 
-    #[inline]
-    pub fn range_mut<R>(&mut self, range: R) -> &mut [(K, V)]
-        where R: RangeBounds<K>
-    {
-        let (start, end) = self.range_slice_indices(range);
-        (&mut self.data[start .. end])
-    }
-
     #[inline]
     pub fn remove_range<R>(&mut self, range: R)
         where R: RangeBounds<K>