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/testdir/test_window_cmd.vim b/src/testdir/test_window_cmd.vim
index 0fe0f11..04a5d10 100644
--- a/src/testdir/test_window_cmd.vim
+++ b/src/testdir/test_window_cmd.vim
@@ -1501,12 +1501,9 @@
     call assert_equal(h0, winheight(0))
     call assert_equal(1, &cmdheight)
   endfor
-  " supports cmdheight=0
-  set cmdheight=0
   call assert_true(win_move_statusline(0, 1))
-  call assert_equal(h0 + 1, winheight(0))
-  call assert_equal(0, &cmdheight)
-  set cmdheight&
+  call assert_equal(h0, winheight(0))
+  call assert_equal(1, &cmdheight)
   " check win_move_statusline from bottom window on top window ID
   let id = win_getid(1)
   for offset in range(5)