[backport#15492 2/2] [cleanup] Remove unused CReserveKey

Summary:
https://github.com/bitcoin/bitcoin/pull/15748/commits/1b46a4889f894d63062454fb9f4c2e88689a9d75

Depends on D6712

Concludes backport of Core PR15748

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6714