patch 8.2.4677: the Athena GUI support is outdated

Problem:    The Athena GUI support is outdated.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/if_mzsch.c b/src/if_mzsch.c
index c24edf3..2c0f39c 100644
--- a/src/if_mzsch.c
+++ b/src/if_mzsch.c
@@ -809,7 +809,7 @@
 #elif defined(FEAT_GUI_GTK)
 static gboolean timer_proc(gpointer);
 static guint timer_id = 0;
-#elif defined(FEAT_GUI_MOTIF) || defined(FEAT_GUI_ATHENA)
+#elif defined(FEAT_GUI_MOTIF)
 static void timer_proc(XtPointer, XtIntervalId *);
 static XtIntervalId timer_id = (XtIntervalId)0;
 #endif
@@ -845,7 +845,7 @@
 # elif defined(FEAT_GUI_GTK)
     static gboolean
 timer_proc(gpointer data UNUSED)
-# elif defined(FEAT_GUI_MOTIF) || defined(FEAT_GUI_ATHENA)
+# elif defined(FEAT_GUI_MOTIF)
     static void
 timer_proc(XtPointer timed_out UNUSED, XtIntervalId *interval_id UNUSED)
 # endif
@@ -853,7 +853,7 @@
     scheme_check_threads();
 # if defined(FEAT_GUI_GTK)
     return TRUE; // continue receiving notifications
-# elif defined(FEAT_GUI_MOTIF) || defined(FEAT_GUI_ATHENA)
+# elif defined(FEAT_GUI_MOTIF)
     // renew timeout
     if (mz_threads_allow && p_mzq > 0)
 	timer_id = XtAppAddTimeOut(app_context, p_mzq,
@@ -868,7 +868,7 @@
     timer_id = SetTimer(NULL, 0, p_mzq, timer_proc);
 # elif defined(FEAT_GUI_GTK)
     timer_id = g_timeout_add((guint)p_mzq, (GSourceFunc)timer_proc, NULL);
-# elif defined(FEAT_GUI_MOTIF) || defined(FEAT_GUI_ATHENA)
+# elif defined(FEAT_GUI_MOTIF)
     timer_id = XtAppAddTimeOut(app_context, p_mzq, timer_proc, NULL);
 # endif
 }
@@ -880,7 +880,7 @@
     KillTimer(NULL, timer_id);
 # elif defined(FEAT_GUI_GTK)
     g_source_remove(timer_id);
-# elif defined(FEAT_GUI_MOTIF) || defined(FEAT_GUI_ATHENA)
+# elif defined(FEAT_GUI_MOTIF)
     XtRemoveTimeOut(timer_id);
 # endif
     timer_id = 0;