patch 8.1.0811: too many #ifdefs

Problem:    Too many #ifdefs.
Solution:   Graduate FEAT_MBYTE, the final chapter.
diff --git a/src/testdir/test_mksession_utf8.vim b/src/testdir/test_mksession_utf8.vim
index 8ffbba2..67af3a9 100644
--- a/src/testdir/test_mksession_utf8.vim
+++ b/src/testdir/test_mksession_utf8.vim
@@ -3,7 +3,7 @@
 set encoding=utf-8
 scriptencoding utf-8
 
-if !has('multi_byte') || !has('mksession')
+if !has('mksession')
   finish
 endif