Cleanup configure.ac to remove old boost versions workarounds

Summary:
Testing against boost versions prior MINIMUM_BOOST_VERSION is no longer
required.

Test Plan:

./autogen.sh
mkdir build && cd build
../configure
make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2255