clean-up: Remove no longer used ivars from CInputCoin

Summary:
This is a partial backport of Core PR12257 : https://github.com/bitcoin/bitcoin/pull/12257/commits/e00b4699cc6d2ee5697d38dd6607eb2631c9b77a

Depends on D3394

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3397