[backport#17381 3/5] Get rid of confusing LegacyScriptPubKeyMan::TopUpKeyPool method

Summary:
Previous discussion https://github.com/bitcoin/bitcoin/pull/17304#discussion_r340307903

https://github.com/bitcoin/bitcoin/pull/17381/commits/491a599b37f3e3a648e52aebed677ca11b0615e2

Depends on D7417

Partial backport of Core PR17381

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7418