FindThreads: Revert "Honor THREADS_PREFER_PTHREAD_FLAG when ... in libc"

Revert !6693 (merged)/!6906 (merged). The check for the -pthread flag can pass on compilers like XL, that interprets it as -p -t hread and returns zero. Prior to that commit, we did not use the check in the CMAKE_HAVE_LIBC_PTHREAD code path. Now we do, it succeeds, and we incorrectly add the -pthread flag for XL.

This change was backported to the 3.21 and 3.22 release series long after they initially came out. Since there may be more cases where we now add -pthread incorrectly, it is simplest to revert the change in all release series pending further investigation.

Fixes: #23270 (closed)
Backport: release