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C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE

Open a Command Prompt in the above folder and then run the delete command like below:
(don't forget the "/DefaultCollection" bit and remember to use quotation marks around your team project name if it has spaces in it)

tfsdeleteproject.exe /force /collection:https://<YourCollection>.VisualStudio.com/DefaultCollection "My project name with spaces"