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/terminal.c b/src/terminal.c
index 52487a3..42a80cd 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -5431,7 +5431,7 @@
 	    attrs = cellattr->attrs;
 	    fg = cellattr->fg;
 	    bg = cellattr->bg;
-	    len = MB_PTR2LEN(p);
+	    len = mb_ptr2len(p);
 	    mch_memmove(mbs, p, len);
 	    mbs[len] = NUL;
 	    p += len;