The Application List is just that, the list of application (detection patterns) that is current from FortiGuard. This is by it' s very nature neither editable nor deleteable (I hope these adjective exist...).
You use one or more application signatures in Application Sensors. You can enter one or several applications or even application categories into a sensor, and then specify this sensor to be used in a policy.
Example: I suppress peer-to-peer apps by default. So, I create a new App sensor and add the ' P2P' category. Unfortunately, Skype belongs to this category but needs to be allowed often. So, I add another entry ABOVE the P2P category, with just ' Skype' as a single application. ' Skype' is allowed, the ' P2P' category is set to be blocked.
Then this sensor is specified in the ' internal' -> ' wan' policy.
"Kernel panic: Aiee, killing interrupt handler!"