Revert "Fix for systems where int64_t == ptrdiff_t" (it was evil and didn't work anyway)

This reverts commit b7b6f8fe.

src/ngraph/dimension.hpp

@@ -73,20 +73,7 @@ namespace ngraph
         }
         /// \brief Convert this dimension to `ptrdiff_t`. This dimension must be static.
         /// \throws std::invalid_argument If this dimension is dynamic.
-        ///
-        /// The type signature here is a bit confusing, so it bears explanation. On some systems,
-        /// `ptrdiff_t` and `int64_t` are equivalent. This means we cannot declare conversion
-        /// operators for both types. On other systems, `ptrdiff_t` and `int64_t` are different,
-        /// so we _have_ to declare a separate conversion operator for `ptrdiff_t`.
-        ///
-        /// On systems where `int64_t` and `ptrdiff_t` are the same type, this definition will be
-        /// disabled. On systems where they are different, it is equivalent to:
-        ///
-        ///    explicit operator ptrdiff_t() const { ... }
-        ///
-        explicit
-            operator std::enable_if<!std::is_same<int64_t, ptrdiff_t>::value, ptrdiff_t>::type()
-                const
+        explicit operator ptrdiff_t() const
         {
             if (is_dynamic())
             {