[libsigcplusplus] Remove now-useless SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE.

[Murray Cumming <murrayc src gnome org>]

commit f978de7906d5fbf9fdfc47341150ecda994e477a
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 1 10:52:47 2016 +0200

    Remove now-useless SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE.

 sigc++/functors/functor_trait.h |   24 ------------------------
 sigc++/functors/slot_base.h     |    8 +++-----
 2 files changed, 3 insertions(+), 29 deletions(-)
---
diff --git a/sigc++/functors/functor_trait.h b/sigc++/functors/functor_trait.h
index 44f0499..a0df4c0 100644
--- a/sigc++/functors/functor_trait.h
+++ b/sigc++/functors/functor_trait.h
@@ -166,30 +166,6 @@ struct functor_trait<T_functor, false, true>
     using functor_type = T_functor;             \
   };
 
-#ifndef SIGCXX_DISABLE_DEPRECATED
-/** Helper macro, if you want to mix user-defined and third party functors with libsigc++.
- *
- * If you want to mix functors not derived from sigc::functor_base with libsigc++,
- * and your compiler can deduce the result type of the functor with the C++11
- * keyword <tt>decltype</tt>, use this macro inside namespace sigc like so:
- * @code
- * namespace sigc {
- *   SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE
- * }
- * @endcode
- *
- * Functors with overloaded operator()() are not supported.
- *
- * @newin{2,2,11}
- *
- * @deprecated This macro does nothing. The test it activated in libsigc++
- *             versions before 2.6, is now unconditionally activated.
- *
- * @ingroup sigcfunctors
- */
-#define SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE // Empty
-#endif // SIGCXX_DISABLE_DEPRECATED
-
 #ifndef DOXYGEN_SHOULD_SKIP_THIS
 // detect the return type and the functor version of non-functor types.
 
diff --git a/sigc++/functors/slot_base.h b/sigc++/functors/slot_base.h
index 6839fdf..2e27f76 100644
--- a/sigc++/functors/slot_base.h
+++ b/sigc++/functors/slot_base.h
@@ -238,11 +238,9 @@ struct SIGC_API slot_do_unbind
  * @endcode
  *
  * If you connect a C++11 lambda expression or a std::function<> instance to
- * a signal or assign it to a slot,
- * - With libsigc++ versions before 2.6, if the return type is not void,
-     you must use the #SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE macro,
- * - if your functor contains references to sigc::trackable derived objects,
- *   those objects will not be tracked, unless you also use sigc::track_obj().
+ * a signal or assign it to a slot, if your functor contains references to
+ * sigc::trackable derived objects, those objects will not be tracked,
+ * unless you also use sigc::track_obj().
  *
  * @ingroup sigcfunctors
  */