Revert "Disable thread_local for i686-mingw"

Summary:
This reverts commit e409be178eae211ac377a1bbddaffa969421cdba.

It is no longer required after D5612.
Depends on D5612.

Test Plan:
Build for windows (64 bits obviously).
Run bitcoin-qt.exe and test_bitcoin.exe.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5635