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