Remove check for thread_local in build systems
Summary: It is part of C++11, so has become useless to check for.
Test Plan:
make check ninja check
Reviewers: #bitcoin_abc, jasonbcox, Fabien
Reviewed By: #bitcoin_abc, jasonbcox, Fabien
Subscribers: jasonbcox, teamcity, Fabien, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2219