Remove fallbacks for boost_filesystem < v3
Minimum boost version is 1.58.0, which means the configure step won't
even pass with older boost.
This version has boost filesystem v3, which means the
(crappy) fallbacks for older versions can go.
Backport of core PR9727
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2242