test: remove unused helpers random_transaction(), make_change() and gather_inputs()

Summary:
This is a backport of core#20159 [2/3]
https://github.com/bitcoin/bitcoin/pull/20159/commits/8ee3536b2b77aeb3a48df5b34effbc7345ef34d8

Depends on D10517

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10518