Hi Jon,
I was having a similar issue in 3.7 and found that your solution didn't work. I am not really a CSS person but with a bit of trial and error messing about with your options, the following worked to remove both the navigation bar and the "jump to" drop down.
body.path-mod.pagelayout-incourse select#jump-to-activity {display: none;}
body.path-mod.pagelayout-incourse nav.activity-navigation {display: none;}Hopefully this helps someone wanting to remove the activity navigation.