Cleanup: Merge QQmlDataLoader and QQmlTypeLoader

There exists only one sub-class of QQmlDataLoader and there are no virtual
functions.

This also clears up a confusion reported in Jira that suggested QQmlDataLoader
should get a virtual destructor as it is being subclassed and somebody might
have a pointer only to a QQmlDataLoader and deleting that would not do proper
polymorphic destruction.

Change-Id: I34611be5a95522bc85a87fb7df97b6d1de7a3d37
Reviewed-by: Michael Brasser <michael.brasser@live.com>
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>
Cleanup: Remove unused code

isConcurrent is not virtual and always returns true, so we can remove
it and remove the if() on the caller side.

Change-Id: Ie750fdeddaf8c339e1fcab99ebd967e3446bd975
Reviewed-by: Michael Brasser <michael.brasser@live.com>