cache-tree API: remove redundant update_main_cache_tree()

Remove the redundant update_main_cache_tree() function, and make its
users use cache_tree_update() instead.

The behavior of populating the "the_index.cache_tree" if it wasn't
present already was needed when this function was introduced in [1],
but it hasn't been needed since [2]; The "cache_tree_update()" will
now lazy-allocate, so there's no need for the wrapper.

1. 996277c520 (Refactor cache_tree_update idiom from commit,
   2011-12-06)
2. fb0882648e (cache-tree: clean up cache_tree_update(), 2021-01-23)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/index-compatibility.cocci

@@ -94,6 +94,9 @@ identifier ACT = active_cache_tree;
 |
 - cache_name_pos
 + index_name_pos
+|
+- update_main_cache_tree
++ cache_tree_update
 )
   (
 + &the_index,