DETROIT — The openers for Mac Saturn’s national tour announced their withdrawal from the tour after FBI agents arrested the Mac Saturn keyboardist on child pornography charges Friday. Mac Saturn keyboardist Evan Daniel Mercer, 30, was arrested hours before the band’s headline show at the Fillmore Detroit on Friday. He now faces several charges including enticement of a minor and producing and/or attempting to produce child pornography. Mac Saturn announced Mercer was “no longer a member of the band” in a social media post shortly after The Detroit News broke the story. The Thing with Feathers,…