Disney sold Power Rangers back to the original owner in 2010. Don't know exactly when they got pulled from the park, but we have pictures of my college-age daughter with several friends & the Power Rangers in the park from Spring Break so they were still there in March 2010. But they have been gone from Hollywood Studios for 2 or 3 years now. Not sure the "violence" factor was any part of the consideration, since they dumped Power Rangers after they acquired the rights to Marvel "super heroes" ...which are a lot more violent, if you think about it. Power Rangers were more campy, stylized fighting than real violence.
Power Rangers were leased to Nickleodeon next, I think.