Remove some useless code in the configure script.

Almost ten years ago, commit e48322a6d6cfce1ec52ab303441df329ddbc04d1 broke
the logic in ACX_PTHREAD by looping through all the possible flags rather
than stopping with the first one that would work.  This meant that
$acx_pthread_ok was no longer meaningful after the loop; it would usually
be "no", whether or not we'd found working thread flags.  The reason nobody
noticed is that Postgres doesn't actually use any of the symbols set up
by the code after the loop.  Rather than complicate things some more to
make it work as designed, let's just remove all that dead code, and thereby
save a few cycles in each configure run.