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Remove 'boost::optional'-related gcc warnings

Description

Remove 'boost::optional'-related gcc warnings

Summary: Backport of core PR15292.

Test Plan:
With GCC:

ninja check

Make sure the warnings are gone (tested with GCC 7.5).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5696