Check if `SiteTree_ImageTracking` is even still valid, and whether it should be ignored in `FileSubsites::augmentSQL()`

[sabina-talipova]

Description

Investigate if there might be a more suitable approach than ignoring sub-sites when a form has the 'SiteTree_ImageTracking' value or we're querying by ID. See

SiteTree_ImageTracking doesn't seem to actually exist - there's only one line in SiteTree that mentions ImageTracking, and it's in reference to the FileTracking relation added to SiteTree in silverstripe/assets.
So most likely this is just dead code that can be removed.

Note:

• ManyManyList::removeAll() changing the from table after FileSubsites::augmentSQL() is called.