[libsigcplusplus] Remove now-irrelevant comment.

[Murray Cumming <murrayc src gnome org>]

commit 717c901426f290e53e26a645a1e914968a0d3964
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Apr 15 12:36:44 2016 +0200

    Remove now-irrelevant comment.

 sigc++/visit_each.h |    4 ----
 1 files changed, 0 insertions(+), 4 deletions(-)
---
diff --git a/sigc++/visit_each.h b/sigc++/visit_each.h
index 2e81a95..30aad54 100644
--- a/sigc++/visit_each.h
+++ b/sigc++/visit_each.h
@@ -53,10 +53,6 @@ struct limit_derived_target
 private:
   using target_type = T_target;
 
-  // This should really be an inner class of limit_derived_target, without the T_limit template type,
-  // But the SUN CC 5.7 (not earlier versions) compiler finds it ambiguous when we specify a
-  // particular specialization of it.
-  // and does not seem to allow us to tell it explicitly that it's an inner class.
   template <class T_type, class T_limit, bool I_derived = is_base_of_or_same_v<typename 
T_limit::target_type, T_type>>
   struct with_type;