QBluetoothSocketBasePrivate: remove unused QObject* ctor arg

It's not used, and I'll want to replace it with a QBluetoothSocket *qq
argument to seed q_ptr, but that is also a QObject, and would match,
so first make sure there are no callers left, by removing the
argument.

Amends 28eeb7afc27e99deeb52eefe10cdd458c7621698 (5.12).

Pick-to: 6.12 6.8
Change-Id: I489c49f40144a20df30abc91add1e4b97178042d
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>