Remove market class from OOPII

[jstac]

I propose that we remove the section https://python-programming.quantecon.org/python_oop.html#example-a-market, since

• the lecture is already long
• we now have a more careful treatment of supply and demand in https://python-programming.quantecon.org/python_oop.html#example-a-market
• we are gradually reducing the number of classes we use, since they are more complex to compile.