[gtkmm] Remove PORTING

[Kjell Ahlstedt <kjellahl src gnome org>]

commit ad03459f1c0d63c194e76fd7310a278131d4aa74
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Tue Nov 6 10:38:13 2018 +0100

    Remove PORTING
    
    It does contain any useful information.

 Makefile.am | 2 +-
 PORTING     | 5 -----
 2 files changed, 1 insertion(+), 6 deletions(-)
---
diff --git a/Makefile.am b/Makefile.am
index 141ad3ca..c4f9c02f 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -46,7 +46,7 @@ pkgconfig_DATA = gdk/$(GDKMM_MODULE_NAME).pc gtk/$(GTKMM_MODULE_NAME).pc
 include $(srcdir)/MSVC_NMake/filelist.am
 include $(srcdir)/win32_installer/filelist.am
 
-text_files = COPYING.tools PORTING README.SUN README.win32
+text_files = COPYING.tools README.SUN README.win32
 msvc_files = $(addprefix MSVC_NMake/,$(msvc_nmake_data))
 winstall_files = $(addprefix win32_installer/,$(windows_installer_files))