You can remove an outcome from a project as long as none of the following criteria apply to any indicators (indicative Progress Trackers) in that outcome.
Criteria
A story has been created in that project using that Progress Tracker
Steps
Go to the Project page
Select More Options from the cover section
A dropdown menu appears
Select Strategy
You arrive on the project’s Strategy page
Press the Edit Goals button
You arrive on the Edit Project Strategy page
From here you can remove an outcome by pressing the Remove button above it
Scroll to the bottom and press Save in order to completely save changes to the project
You arrive on the Project Targets page
Scroll to the bottom and press Save
You arrive on the Project page