ChangeSet::removeObject() should handle case of implicit added item being removed.

[sabina-talipova]

Description

Changesets can have items added implicitly. In that scenario, there may be no ChangeSetItem record in the database, so the current implementatio of removeObject() won't remove that implicit changeset item.

We should add logic to this method to check for implicit changeset items, and mark them as removed so they're not included in the actionable changeset.

See

silverstripe-versioned/src/ChangeSet.php

Lines 215 to 219 in 5359490

	if ($item) {
	// TODO: Handle case of implicit added item being removed.
	$item->delete();
	}