The problem is that will remove application but not deployed over the air. Or just after a moment application over the air will be deployed once again.
An Issue was opened same as yours had the following reponse:
Currently, there is no way to delete the feeds and cloud config once set. I suggest you, please create a feedback item on the UserVoice and upvote. All the feedback you share in these forums will be monitored and reviewed by the Microsoft engineering teams responsible for building Azure.
So please go to UserVoice in link above and upvote and/or enter your concerns.
azsphere feed list
Listing all feeds.
--> [3369f0e1-dedf-49ec-a602-2aa98669fd61] 'Retail Azure Sphere OS'
--> [82bacf85-990d-4023-91c5-c6694a9fa5b4] 'Retail Evaluation Azure Sphere OS'
--> [8c24c679-0d0b-47ec-adad-480a6acd7e0f] 'ASTestFeed03'
--> [96bb8dd0-6d04-418b-bdc7-e5b4a34df342] 'ASTestFeed0'
--> [a1463e59-fbbe-4a2c-81de-c83c91a0bae1] 'ASTestFeed100'
but how to remove applications assigned to feeds?