visual studio 2019 remove an azure devops connection

You can right-click the repo you want to remove, and then select the Remove option. It should be noted that once a repo is added, VS cannot remove the repo by default unless there is another repo, e.g in the figure below, I also have a local repository below. You have to switch to another repository to remove the current one, otherwise the displayed remove option will be unusable.

Repeating the comment below because it's easier:

If you go to Team in the menu and select 'Disconnect from Server', then you are allowed to delete a connection without the need of creating a new connection"