QtHelp: Unregister documentation atomically

unregisterDocumentation() deleted the NamespaceTable row twice, the
first time before looking up the namespace's folder, and didn't use a
transaction. For a namespace without a FolderTable row, it deleted the
namespace and then returned false, leaving the index data of the
namespace orphaned in the collection file for good. The same happened
when any of the later statements failed.

Remove the first DELETE statement, wrap the removal in a transaction
and check the result of the commit.

Task-number: QTBUG-150686
Pick-to: 6.12 6.8
Change-Id: I4637d84583e6a5d7f13a569ef08494beb543cb34
Reviewed-by: Kai Köhne <kai.koehne@qt.io>