wallet: Remove boost from PeriodicFlush
Summary:
The boost::this_thread::interruption_point() in the code base currently block the replacement of boost::thread with std::thread. [1] Remove them from the wallet because they are either unused or useless.
Backport of core PR18792.
Depends on D8615.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8616