wallet: Never schedule MaybeCompactWalletDB when -flushwallet is off

Summary:

User-facing, this is a refactor. Internally, the scheduler does not have
to call a mostly empty function every half a second.

Backport of core PR18923.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8638