fixincludes, Darwin: Remove handling for attribute availability.

Since we have an implementation we no longer need to work around
cases where support for absence was broken.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

fixincludes/fixincl.x

@@ -2,11 +2,11 @@
  *
  * DO NOT EDIT THIS FILE   (fixincl.x)
  *
- * It has been AutoGen-ed  May 22, 2024 at 10:16:02 AM by AutoGen 5.18.7
+ * It has been AutoGen-ed  June  1, 2024 at 05:24:18 PM by AutoGen 5.18.7
  * From the definitions    inclhack.def
  * and the template file   fixincl
  */
-/* DO NOT SVN-MERGE THIS FILE, EITHER Wed May 22 10:16:02 BST 2024
+/* DO NOT SVN-MERGE THIS FILE, EITHER Sat Jun  1 17:24:18 BST 2024
  *
  * You must regenerate it.  Use the ./genfixes script.
  *