You can adjust which tracks appear as selectable options in the left side bar of the Plan page via the updated Gym Tracks page. To access that page click on the "Manage" link located under the tracks list. 

If you want to remove a track from the Plan page, toggle the "Active" setting to off. 

Doing so will move that track to the Inactive tracks table. Making a track "Inactive" does NOT delete the track. It simply removes it from the Plan page. Note, it also hides that track from any members who might be following it.  

Any track set to inactive will no longer be visible in the tracks list on the Plan page calendar view.