Programmatically remove content from a group
I believe that function was overlooked by the developers. In the issues queue for the Group module, which is what your error message refers to, I can see that the inverse of addMember(), removeMember(), was added later in the project's life. It seems reasonable to suppose that the inverse of addContent() was also overlooked, with the only difference being that it has remained unimplemented to this day.

There are no functions with names hinting at content-removing behaviour evident in the module's GroupInterface definition, so I recommend opening a new issue with the project to let them know about the gap.