Merge #12284: Remove assigned but never used local variables. Enable linter checking for unused local variables.

Summary:
ea04bf7862 Enable flake8 warning F841 ("local variable 'foo' is assigned to but never used") (practicalswift)
169f3e8637 Remove assigned but never used local variables (practicalswift)

Pull request description:

Remove assigned but never used local variables. Enable Travis checking for unused local variables.

Tree-SHA512: d6052ec9044c5d1f03d874ea3c8addd5a156779213ef9200f89d3ae53230f2fd1691aff405c3dae14178e5ef09912c4432e92f606ef4a5220ed9daa140cdee81

Backport of Core PR12284
https://github.com/bitcoin/bitcoin/pull/12284/

Note: rather than Travis checking for unused local variables, our linter will check for unused local variables.

Test Plan:

test_runner.py --extended
arc lint --everything

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3814