Merge #11683: tests: Remove unused mininode functions {ser,deser}_int_vector(...). Remove unused imports.

Summary:
f522fb7c9 tests: Remove unused mininode functions deser_int_vector(f) and ser_int_vector(l) (practicalswift)

Pull request description:

• Remove unused mininode functions deser_int_vector(f) and ser_int_vector(l). Last use removed in 3858aabbd084b2cc4534c40a178bbcc22fb82968. Friendly ping @jnewbery :-)
• Remove unused imports.

Tree-SHA512: 840c5623eae9f929561f6e86816883c5904ec1af82fc8d5e56dee1c0b1fe22e8600c10f7358ed8b556b3aec8c65c4910f6eee30e8015a573c4df8bef91124d3e

Backport of Core PR11683
https://github.com/bitcoin/bitcoin/pull/11683/commits/f522fb7c9c121641ce552774e3993942106ec336

This PR actually had two commits, but the second commit (0f3b752ec Remove unused imports (practicalswift)) was blank after fixing merge conflicts.

Test Plan:

make check
test_runner.py

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

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3205