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

Revert commit 5efb6fb516 (FindThreads: Honor THREADS_PREFER_PTHREAD_FLAG
when pthread is found in libc, 2021-11-03, v3.21.5~4^2).  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