Approve functions used as default arguments (#4699)

* Approve functions used as default argumenets

* The default value for **seed** is the result of a function call

The default value for **seed** is the result of a function call which is not normally recommended and causes flake8-bugbear to raise a B008 error. However, in this case, it is accptable because `LinearCongruentialGenerator.__init__()` will only be called once per instance and it ensures that each instance will generate a unique sequence of numbers.

* The default value for **backend** is the result of a function call

The default value for **backend** is the result of a function call which is not normally recommended and causes flake8-bugbear to raise a B008 error. However, in this case, it is accptable because `Aer.get_backend()` is called when the function is definition and that same backend is then reused for function calls.

* Update linear_congruential_generator.py

* Update ripple_adder_classic.py

* Update ripple_adder_classic.py

* Update ripple_adder_classic.py

* Update ripple_adder_classic.py

* Update ripple_adder_classic.py