patch 8.1.1079: no need for a separate ScreenLinesUtf8() test function

Problem:    No need for a separate ScreenLinesUtf8() test function.
Solution:   Get the composing characters with ScreenLines().
diff --git a/src/testdir/test_listchars.vim b/src/testdir/test_listchars.vim
index 1e90856..8ddfc7f 100644
--- a/src/testdir/test_listchars.vim
+++ b/src/testdir/test_listchars.vim
@@ -130,7 +130,7 @@
         \ ]
   redraw!
   call cursor(1, 1)
-  let got = ScreenLinesUtf8(1, virtcol('$'))
+  let got = ScreenLines(1, virtcol('$'))
   bw!
   call assert_equal(expected, got)
   let &encoding=oldencoding