Start by writing down what those methods should do and work out which class they should be in. I can't tell that because I don't know how you designed your application. I don't think that simply removing them from a customer's list of bicycles will be sufficient, however.
What you have could be improved greatly. There is no need to call toString() in line 3; System.out.print calls toString() indirectly, and so does the + operator. Why are you only showing the List when it contains bicycles? A java.util.List can usually be printed directly, and if empty it simply displays “”.
Don't use multiple dot operators (line 5) if you can help it. The rents variable should have private access and the Customer class should have a method like hasRents().tt
How many bikes do you have in Inventory (Perhaps an ArrayList)?
Will you track customers?
What exactly do you want the application to do?
What is the input? What is the output?
I think it would be a good idea to figure out what you want. Sketch is out on paper. I have been following Mr. Ritchies example of writing what I want the application to accomplishment.
Best - Two Cent Steve