[backport#15853] wallet: Remove unused import checkpoints.h

Summary:
fadf7d1390 wallet: Remove unused import checkpoints.h (MarcoFalke)

Pull request description:

Yet another silent merge conflict. This one was caused by unsorted includes.

https://github.com/bitcoin/bitcoin/pull/15853/commits/fadf7d13905d76ef4d2d8b9b96a6b9269f78fbba

Backport of Core PR15853

Test Plan:

ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7029