My first question was whether FileBird was really under GPLv2; they don't exactly go out of the way to make the license a salient. But after some digging it looks like you're right.
That given, what you propose - removing the billing and registration hooks - is completely lawful. You don't even have to use another name, though it would probably be kind of you to do so as it may reduce confusion. You will of course need to release your version under GPLv2 also, which may impact on your plans to "sell it off".
Be prepared for the possibility of opprobrium: some people release software under copyleft licences, possibly without fully understanding that that empowers people to develop the code in ways they themselves would not choose to. They then get very upset when other people try to exercise the rights they've explicitly been given. None of that, however, makes what you propose in any way unlawful.