patch 8.1.0811: too many #ifdefs

Problem:    Too many #ifdefs.
Solution:   Graduate FEAT_MBYTE, the final chapter.
diff --git a/src/configure.ac b/src/configure.ac
index 2dcfe46..e5525ea 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -2124,9 +2124,7 @@
 	[  --enable-multibyte      Include multibyte editing support.], ,
 	[enable_multibyte="yes"])
 AC_MSG_RESULT($enable_multibyte)
-if test "$enable_multibyte" = "yes"; then
-  AC_DEFINE(FEAT_MBYTE)
-else
+if test "$enable_multibyte" != "yes"; then
   AC_MSG_ERROR([The multi-byte feature can no longer be disabled. If you have
 		a problem with this, discuss on the Vim mailing list.])
 fi