Remove fallbacks for boost_filesystem < v3

Summary:
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

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2242