Remove object::borrowed/stolen

PR #771 deprecated them as they can cause linking failures (#770), but
the deprecation tags cause warnings on GCC 5.x through 6.2.x.  Removing
them entirely will break backwards-compatibility consequences, but the
effects should be minimal (only code that was inheriting from `object`
could get at them at all as they are protected).

Fixes #777

include/pybind11/pytypes.h

@@ -236,11 +236,6 @@ protected:
     // Tags for choosing constructors from raw PyObject *
     struct borrowed_t { };
     struct stolen_t { };
-    // These can cause linkage problems; see #770
-    PYBIND11_DEPRECATED("Use of the `borrowed` static variable is deprecated; use `borrowed_t{}' instead")
-    static constexpr borrowed_t borrowed{};
-    PYBIND11_DEPRECATED("Use of the `stolen` static variable is deprecated; use `stolen_t{}' instead")
-    static constexpr stolen_t stolen{};
 
     template <typename T> friend T reinterpret_borrow(handle);
     template <typename T> friend T reinterpret_steal(handle);