Remove workaround for libstdc++4.8 in linear_map

The minimum version for the library is upgraded to 4.9.
Issue #142.

src/linear_map.h

@@ -44,17 +44,10 @@ struct DefaultEraser {
   /// @returns The iterator as the result of call to the container's ``erase``.
   ///
   /// @{
-  template <class T>
-  static typename T::iterator erase(typename T::iterator it, T* container) {
+  template <class T, class Iterator>
+  static typename T::iterator erase(Iterator it, T* container) {
     return container->erase(it);
   }
-  template <class T>  // Workaround for the C++11 bug in libstdc++ 4.8.
-  static typename T::iterator erase(typename T::const_iterator it,
-                                    T* container) {
-    return DefaultEraser::erase(
-        std::next(container->begin(), std::distance(container->cbegin(), it)),
-        container);
-  }
   /// @}
 };