[glibmm] Gio::ActionMap: Remove TODO about deriving from ActionGroup.

[Murray Cumming <murrayc src gnome org>]

commit e94cdf82ded62dd54c87189e8151bb30dea93a7d
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Dec 2 12:41:17 2016 +0100

    Gio::ActionMap: Remove TODO about deriving from ActionGroup.
    
    Because the requirement was removed from GActionMap in 2013:
    https://git.gnome.org/browse/glib/commit/gio/gactionmap.c?id=390115f3851de49f93918cda9266ab857382cc3e

 gio/src/actionmap.hg |    4 ----
 1 files changed, 0 insertions(+), 4 deletions(-)
---
diff --git a/gio/src/actionmap.hg b/gio/src/actionmap.hg
index e3bcdf0..518c089 100644
--- a/gio/src/actionmap.hg
+++ b/gio/src/actionmap.hg
@@ -31,10 +31,6 @@ namespace Gio
 
 class Action;
 
-//TODO: Instead derive from ActionGroup, when we can break ABI,
-//because the GActionMap interface requires the GActionGroup interface.
-//LoadableIcon does a similar thing correctly, for instance.
-
 /** ActionMap - Interface for action containers.
  * The ActionMap interface is implemented by ActionGroup implementations that
  * operate by containing a number of named Action instances, such as