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/ex_docmd.c b/src/ex_docmd.c
index 1891d80..0d52f58 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -8370,14 +8370,9 @@
     // After drawing the statusline screen_attr may still be set.
     screen_stop_highlight();
 
-#ifdef HAS_MESSAGE_WINDOW
-    if (!use_message_window())  // append messages in the message window
-#endif
-    {
-	// Reset msg_didout, so that a message that's there is overwritten.
-	msg_didout = FALSE;
-	msg_col = 0;
-    }
+    // Reset msg_didout, so that a message that's there is overwritten.
+    msg_didout = FALSE;
+    msg_col = 0;
 
     // No need to wait after an intentional redraw.
     need_wait_return = FALSE;