meson: remove obsolete check for EPOLL_CLOEXEC

meson: remove obsolete check for EPOLL_CLOEXEC

The EPOLL_CLOEXEC constant was introduced to Linux in

  commit a0998b50c3f0b8fdd265c63e0032f86ebe377dbf
  Author: Ulrich Drepper <drepper@redhat.com>
  Date:   Wed Jul 23 21:29:27 2008 -0700

    flag parameters: epoll_create

This is old enough that all our supported platforms can be assumed
to have this feature. For added fun this whole meson check was
semantically insane because EPOLL_CLOEXEC is not a valid arg
to unshare().

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>