The only way to “hide” the Assignments and Progress icon is to either remove the plan completely or remove the book(s) from the plan. This affects everyone on the project so it should be done carefully. I believe that the Assignments and Progress tool can be very useful if used correctly so I would be very hesitant to remove it.
To remove books from the plan you must go to Project > Project Properties and Settings > Books tab. This tab is used to indicate which books should be included in the Assignments and Progress. If you have a plan you must have at least one book selected here.