docs: remove reference to src/tools/thread
This directory and the ability to build the thread test independently
were removed in commit
8a2121185b.
Reported-by: e.indrupskaya@postgrespro.ru
Discussion: https://postgr.es/m/
160379609706.24746.
7506163279454026608@wrigleys.postgresql.org
Backpatch-through: 9.5