Remove RubyMotion trail

My goal with this commit is to present a more opinionated trail map for
students. thoughtbot currently writes our iOS applications in
Objective-C. That's where our expertise now resides, and is where we can
provide the most guidance. I think it's important to keep the trails
focused and not over-burden learners with choices. Our job is primarily
as curators. So, I'm pruning this trail out to focus the Learn offering
and reduce our maintenance and support burden.

If and when this is merged, I believe we'd want to delete the RubyMotion trail on Learn and remove the "related topic" link from the iOS trail on Learn. I don't know if we want to do anything else regarding migrating data for students who have completed RubyMotion trails or reach out to them with an email linking to the RubyMotion resources so they have a copy of them outside of the Learn product. /cc @cpytel.