patch 8.1.2120: some MB_ macros are more complicated than necessary

Problem:    Some MB_ macros are more complicated than necessary. (Dominique
            Pelle)
Solution:   Simplify the macros.  Expand inline.
diff --git a/src/ex_getln.c b/src/ex_getln.c
index ef87413..838bd84 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -2781,7 +2781,7 @@
 		    }
 		    else
 		    {
-			len = MB_PTR2LEN(p);
+			len = mb_ptr2len(p);
 			msg_outtrans_len(p, len);
 			vcol += ptr2cells(p);
 			p += len;