Hiding a project
To hide a project, simply select from the dropdown menu:

This project and its sub-projects will disappear from the left menu.
Hiding projects is just a personal display setting, therefore it is not applied for other users.
Showing hidden projects
To show hidden projects, click the plus sign icon on the Projects pane, then choose :

You will see all hidden projects like this:

From here, you will be able to show these projects again by clicking from the dropdown menu:

To show a hidden sub-project, you should show its parent first.