Remove redundant variables, statements and forward declarations

Summary:
Remove redundant …

• ~access modifiers,~
• forward declarations,
• unused variables,
• statements, and
• ~return types from lambdas.~

Backport of Bitcoin Core PR13691
https://github.com/bitcoin/bitcoin/pull/13691

Test Plan:

make check-all

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4142