Remove tests against BOOST_VERSION older than MINIMUM_REQUIRED_BOOST

Summary:
The build will fail if boost version is < MINIMUM_REQUIRED_BOOST.
These checks are no longer required.

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2243