patch 9.0.0340: the 'cmdheight' zero support causes too much trouble

Problem:    The 'cmdheight' zero support causes too much trouble.
Solution:   Revert support for 'cmdheight' being zero.
diff --git a/src/popupwin.c b/src/popupwin.c
index 2f9df8d..8292dba 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -29,7 +29,7 @@
 };
 
 #ifdef HAS_MESSAGE_WINDOW
-// Window used for messages when 'winheight' is zero.
+// Window used for ":echowindow"
 static win_T *message_win = NULL;
 #endif
 
@@ -4529,16 +4529,6 @@
 	popup_hide(message_win);
 }
 
-/*
- * If the message window exists: close it.
- */
-    void
-popup_close_message_win(void)
-{
-    if (message_win != NULL)
-	popup_close(message_win->w_id, TRUE);
-}
-
 #endif
 
 /*