patch 8.2.4677: the Athena GUI support is outdated

Problem:    The Athena GUI support is outdated.
Solution:   Remove the Athena GUI code.
diff --git a/src/testdir/setup_gui.vim b/src/testdir/setup_gui.vim
index 05e4999..2e5e777 100644
--- a/src/testdir/setup_gui.vim
+++ b/src/testdir/setup_gui.vim
@@ -1,6 +1,6 @@
 " Common preparations for running GUI tests.
 
-let g:x11_based_gui = has('gui_athena') || has('gui_motif')
+let g:x11_based_gui = has('gui_motif')
 	\ || has('gui_gtk2') || has('gui_gnome') || has('gui_gtk3')
 
 " Reasons for 'skipped'.