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