Merge #11044: [wallet] Keypool topup cleanups

Summary:
67ceff4 [wallet] Add logging to MarkReserveKeysAsUsed (John Newbery)
1221f60 [wallet] Remove keypool_topup_cleanups (John Newbery)

Pull request description:

A couple of minor cleanups suggested by @ryanofsky here: https://github.com/bitcoin/bitcoin/pull/11022#pullrequestreview-55598940

Does not affect functionality. Not required for v0.15.

Tree-SHA512: d8d0698fd26ea49a4157e68669d5511095760c3a1ecfa3f917e3f273efbafb55c51a202d677614216eae3f796b6e8d17506b2ec2d4799a94f18981b396e65eec

Backport of Core PR11044
https://github.com/bitcoin/bitcoin/pull/11044/

Test Plan:

make check
test_runner.py

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

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3998