Deprecate Aggregate and DataObject::getComponentsQuery

https://github.com/silverstripe/silverstripe-framework/blob/3.1/model/Aggregate.php seems like it could be a bit of a relic of 2.4. It references DataObject::getManyManyComponentsQuery (which no longer exists), and it seems that most of its functionality is superceded by DataList, other than the caching behaviour.

In addition, this seems to be the only place where `DataObject::getComponentsQuery` is used. Is there any situation where this function is useful where `DataList` or `DataObject::getComponents` can't do the job?

Can we deprecate these, or am I missing a valid reason for their still existing?

I couldn't find any test case coverage or documentation for these, so I didn't bother to attempt 'fixing' `Aggregate`.