unfortunately we cannot delete a team project from visual studio or online control panel. to delete a team project you need to use the command line utility.

run visual studio tools command prompt tools. make sure you open the x86 version as x64 version wouldnt work.

and then run this command:

tfsdeleteproject /collection:https://YOUR-ACCOUNT-URL.visualstudio.com/DefaultCollection "Your project name"

if required this will prompt you to login with a Microsoft Account.

after you log in it will prompt you with a yes/no question. enter y press enter and it will delete your site collection.