patch 8.2.4677: the Athena GUI support is outdated

Problem:    The Athena GUI support is outdated.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/clipboard.c b/src/clipboard.c
index 9eace3d..fbeee72 100644
--- a/src/clipboard.c
+++ b/src/clipboard.c
@@ -1354,7 +1354,7 @@
 
 /*
  * Open the application context (if it hasn't been opened yet).
- * Used for Motif and Athena GUI and the xterm clipboard.
+ * Used for Motif GUI and the xterm clipboard.
  */
     void
 open_app_context(void)