patch 8.0.1236: Mac features are confusing

Problem:    Mac features are confusing.
Solution:   Make feature names more consistent, add "osxdarwin". Rename
            feature flags, cleanup Mac code. (Kazunobu Kuriyama, closes #2178)
diff --git a/src/gui_mac.c b/src/gui_mac.c
index bbb0857..93feabe 100644
--- a/src/gui_mac.c
+++ b/src/gui_mac.c
@@ -267,9 +267,7 @@
 /*  {XK_Help,		'%', '1'}, */
 /*  {XK_Undo,		'&', '8'}, */
 /*  {XK_BackSpace,	'k', 'b'}, */
-#ifndef MACOS_X
-    {vk_Delete,		'k', 'b'},
-#endif
+/*  {vk_Delete,		'k', 'b'}, */
     {vk_Insert,		'k', 'I'},
     {vk_FwdDelete,	'k', 'D'},
     {vk_Home,		'k', 'h'},