What defines a load bearing wall

If you have a reciprocating saw (aka "Sawzall") with a metal or demolition blade it should make short work of the nails. Make your cuts at both ends of the vertical studs, between the stud and the header/footer. Once the nails are cut the vertical studs should come right out. You can probably just use a pry bar to pull off the header.

OK normally load bearing walls will have 2 plates on top. But having 2 plates means nothing.

People frame however they learned to or want. Checking door header also means nothing. Some people flip all door headers - doesn't make the wall load bearing. You can never get into the head of the guy who framed your house. I worked for a construction company in college... the owner taught everyone to basically frame every wall like it was load bearing - 2 top plates and headers flipped was a minimum. I asked him one day why we were doing this and he basically said - Well it might cost an extra $10 on some walls but we never have to worry or redo anything.

You told me this wall goes in same direction as attic joists... That is a good sign. You need to get into the attic and see what it is attached to. If it isn't supporting a joist it isn't load bearing. If this wall is just floating between 2 joists - even with a few 2x4s for cross bracing - it is not load bearing.

However if there is structural bracing in the attic it is/could be load bearing. Or if it is directly under a joist.

You have two choices to figure this out. Get up in the attic or tear down enough drywall to figure out what you have. If you are a gambler and want this wall down just tear out the drywall - you will have to remove it to the joist on each side to redrywall anyway.