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/filepath.c b/src/filepath.c
index 5ffb8c9..3a06eec 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -2613,9 +2613,9 @@
 		&& !(cx == '/' && cy == '\\')
 		&& !(cx == '\\' && cy == '/')))
 	    break;
-	len -= MB_PTR2LEN(px);
-	px += MB_PTR2LEN(px);
-	py += MB_PTR2LEN(py);
+	len -= mb_ptr2len(px);
+	px += mb_ptr2len(px);
+	py += mb_ptr2len(py);
     }
     if (len == 0)
 	return 0;
@@ -3769,14 +3769,14 @@
 		    : c1 - c2;  // no match
 	}
 
-	i += MB_PTR2LEN((char_u *)p + i);
-	j += MB_PTR2LEN((char_u *)q + j);
+	i += mb_ptr2len((char_u *)p + i);
+	j += mb_ptr2len((char_u *)q + j);
     }
     if (s == NULL)	// "i" or "j" ran into "maxlen"
 	return 0;
 
     c1 = PTR2CHAR((char_u *)s + i);
-    c2 = PTR2CHAR((char_u *)s + i + MB_PTR2LEN((char_u *)s + i));
+    c2 = PTR2CHAR((char_u *)s + i + mb_ptr2len((char_u *)s + i));
     // ignore a trailing slash, but not "//" or ":/"
     if (c2 == NUL
 	    && i > 0