patch 8.1.1528: popup_any_visible() is unused

Problem:    Popup_any_visible() is unused.
Solution:   Remove it.
diff --git a/src/version.c b/src/version.c
index 1356fda..aa3986d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1528,
+/**/
     1527,
 /**/
     1526,