[TEST] Remove useless trailing parenthesis in class declaration

Summary:
This probably comes from old python 2 memories, but this is not necessary, never was and is confusing at best so trim it off.

There is no change in behavior.

Test Plan:

ninja check-functional-extended

cd test/functional
git grep -nE "class [^(]+\(\):"

Check there is no remaining case.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11118