[libsigcplusplus] slot_rep::dup(): Remove an unnecessary static_cast.

[Murray Cumming <murrayc src gnome org>]

commit b71b38ca8cac858f3a0c1358f489fd1ecba41fc4
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Jul 28 10:35:53 2016 +0200

    slot_rep::dup(): Remove an unnecessary static_cast.
    
    Found by CLion's code inspection.

 sigc++/functors/slot_base.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/sigc++/functors/slot_base.h b/sigc++/functors/slot_base.h
index 02c3d50..ba7baa8 100644
--- a/sigc++/functors/slot_base.h
+++ b/sigc++/functors/slot_base.h
@@ -100,7 +100,7 @@ public:
    */
   inline slot_rep* dup() const
   {
-    return static_cast<slot_rep*>((*dup_)(const_cast<slot_rep*>(this)));
+    return (*dup_)(const_cast<slot_rep*>(this));
   }
 
   /** Set the parent with a callback.