Remove configure-time thread safety checking (thread_test.c).

Remove configure-time thread safety checking (thread_test.c).

This testing was useful when it was written, nigh twenty years ago,
but it seems fairly pointless for any platform built in the last
dozen or more years.  (Compare also the comments at 8a2121185.)
Also we now have reports that the test program itself fails under
ThreadSanitizer.  Rather than invest effort in fixing it, let's
just drop it, and assume that the few people who still care
already know they need to use --disable-thread-safety.

Back-patch into v14, for consistency with 8a2121185.

Discussion: https://postgr.es/m/CADhDkKzPSiNvA3Hyq+wSR_icuPmazG0cFe=YnC3U-CFcYLc8Xw@mail.gmail.com