tree-walk: drop MAX_TRAVERSE_TREES macro

Since the previous commit dropped the hard-coded limit in
traverse_trees(), we don't need this macro there anymore (the code can
handle any number of trees in parallel).

We do define MAX_UNPACK_TREES using MAX_TRAVERSE_TREES, due to
5290d45134 (tree-walk.c: break circular dependency with unpack-trees,
2020-02-01). So we can just directly define that as "8" now; we know
traverse_trees() can handle whatever we throw at it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-walk.h

@@ -6,8 +6,6 @@
 struct index_state;
 struct repository;
 
-#define MAX_TRAVERSE_TREES 8
-
 /**
  * The tree walking API is used to traverse and inspect trees.
  */

unpack-trees.h

@@ -7,7 +7,7 @@
 #include "string-list.h"
 #include "tree-walk.h"
 
-#define MAX_UNPACK_TREES MAX_TRAVERSE_TREES
+#define MAX_UNPACK_TREES 8
 
 struct cache_entry;
 struct unpack_trees_options;