patch 8.1.0882: checking for FEAT_MBYTE which doesn't exist anymore

Problem:    Checking for FEAT_MBYTE which doesn't exist anymore. (Christ van
            Willegen)
Solution:   Remove it.
diff --git a/src/fileio.c b/src/fileio.c
index 446ae7e..c1aa1a7 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -7471,7 +7471,7 @@
 	    if (*p == '\\')
 		*p = '/';
 
-#if defined(FEAT_MBYTE) && defined(WIN3264)
+#if defined(WIN3264)
     if (enc_utf8)
     {
 	int	len;