Remove latch.c workaround for Linux < 2.6.27.

Remove latch.c workaround for Linux < 2.6.27.

Commit 82ebbeb0 added a workaround for systems with no epoll_create1()
and EPOLL_CLOEXEC.  Linux < 2.6.27 and glibc < 2.9 are long gone.  Now
seems like a good time to drop the extra code, because otherwise we'd
have to add similar already-dead workaround code to new patches using
XXX_CLOEXEC flags that arrived in the same kernel release.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKGKL_%3DaO%3Dr30N%3Ds9VoDgTqHpRSzePRbA9dkYO7snc7HsxA%40mail.gmail.com