Go to My Music > Projects. On the right-hand side of each project, click on the three dots and select the option to 'Delete unused recorded loops'. This will delete all vocal recordings within that project which are not currently used in any part of the track.
Go to My content > Projects. Click on the 'i' symbol on the right-hand side of a project. This will open a menu and, if there are unused vocal recordings in the project, you will see the option to 'Delete unused recorded loops'. Select this and then click 'Delete'.