patch 8.2.4610: some conditions are always true

Problem:    Some conditions are always true.
Solution:   Remove the useless conditions. (closes #9993)
diff --git a/src/message.c b/src/message.c
index af91bd8..2499723 100644
--- a/src/message.c
+++ b/src/message.c
@@ -1803,7 +1803,7 @@
 	int len = (*mb_ptr2len)(str);
 
 	// For multi-byte characters check for an illegal byte.
-	if (has_mbyte && MB_BYTE2LEN(*str) > len)
+	if (MB_BYTE2LEN(*str) > len)
 	{
 	    transchar_nonprint(curbuf, buf, c);
 	    *sp = str + 1;