This is a super old question, but for anybody else who may stumble on this question, I found the answer.
Open TestFlight.
Depending on your device model, long or deep press the app you want to remove.
Click the small preview window that opens.
Scroll to the bottom of the window.
The last link should say Stop Testing.
Click Stop Testing.
This is as of Friday, July 9, 2021. I have an iPhone 12 Pro Max running iOS 14.6.
Hope this helps someone. :)
SnackyStacky
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