From what was shown in the movie I've always assumed Obi-Wan willed himself into the Force just before Vader got him.

As far as graphic violence goes the first Star Wars movie showed the results of Obi-Wan slicing off the arm of an alien and the charred skeletal remains of the elderly couple taking care of Luke. So I'm not thinking this was cut due to how graphic it was.

I'm guessing they didn't go for this because it looked too much like Vader killing Obi-Wan and not Obi-Wan willing himself into the Force. Not a big difference, but enough that Lucas felt it needed changing. Although Obi-Wan's cloak probably should have suffered some type of burns in the movie we saw.