Remove useless include of boost thread
Summary: We probably want to get rid of it at some point now that we are using C++11 but in the meantime, we can limit the spread of the infection.
Test Plan:
make check
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D3042