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_utf8.vim b/src/testdir/test_utf8.vim
index 98793c3..80ca8b5 100644
--- a/src/testdir/test_utf8.vim
+++ b/src/testdir/test_utf8.vim
@@ -91,7 +91,7 @@
   call assert_equal("", screenstring(1, 4))
   call assert_equal("\u3046\u3099", screenstring(1, 5))
 
-  call assert_equal([text . '  '], ScreenLinesUtf8(1, 8))
+  call assert_equal([text . '  '], ScreenLines(1, 8))
 
   bwipe!
 endfunc