Migrate MakeUnique to c++14 std::make_unique
Summary:
Remove the temporary MakeUnique definition.
Depends on D2462
Test Plan:
make check
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2463