[Part 1 of 2] [qa] receivedby.py cleanup and fixes

Summary:
[qa] Tidy up receivedby.py

• Fix flake8 warnings
• Remove the useless get_sub_array_from_array() function
• Reduce runtime for receivedby.py by about half by only using two nodes

Backport of Core PR11055

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1983