Deprecate show_in_sitetree in favour of core hide_from_cms_tree setting

[sminnee]

Lumberjack::stageChildren() is unnecessary. It implements 2 configuration options that are duplicates of built-in features in https://github.com/silverstripe/silverstripe-framework/blob/4/src/ORM/Hierarchy/Hierarchy.php#L420

Hide certain page types in CMS tree

Covered by Hierarchy.hide_from_cms_tree

Hide certain page types in hierarchy

Covered by Hierarchy.hide_from_hierarchy

So, we should deprecate use of Lumberjack.hide_from_hierarchy and Lumberjack.hide_from_cms_tree, and in the next major release of the Lumberjack module remove these fields and the custom stageChildren() method altogether.

Note that as of 4.3, these methods will cease to work reliably unless the Lumberjack extension is applied to SiteTree, due to performance-optimisation of the batch queries to Hierarchy::numChildren().