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/spell.c b/src/spell.c
index de478a3..607bb39 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -2621,7 +2621,7 @@
 
     if (has_mbyte)
     {
-	l = MB_PTR2LEN(p);
+	l = mb_ptr2len(p);
 	s = p;
 	if (l == 1)
 	{