UPDATE:
Time has changed, you can now remove (expire) TestFlight Builds as in this answer but you still cannot delete the build.
OLD:
I asked apple and here is their answer:
I understand you would like to remove a build from iTunes Connect as
shown in your screenshot.Please be advised this is expected behavior as you can remove a build
from being the current build but you cannot delete it from iTunes
Connect. For more information, please refer to the iTunes Connect
Developer Guide:
https://developer.apple.com/library/content/documentation/LanguagesUtilities/Conceptual/iTunesConnect_Guide/
So i just can’t.