patch 8.1.0810: too many #ifdefs

Problem:    Too many #ifdefs.
Solution:   Graduate FEAT_MBYTE, part 4.
diff --git a/src/os_mac.h b/src/os_mac.h
index c858a62..692e253 100644
--- a/src/os_mac.h
+++ b/src/os_mac.h
@@ -37,9 +37,7 @@
 # include <Memory.h>
 # include <OSUtils.h>
 # include <Files.h>
-# ifdef FEAT_MBYTE
-#  include <Script.h>
-# endif
+# include <Script.h>
 #endif
 
 /*