meson: remove obsolete check for LO_FLAGS_AUTOCLEAR

meson: remove obsolete check for LO_FLAGS_AUTOCLEAR

The LO_FLAGS_AUTOCLEAR constant was introduced to Linux in

  commit 96c5865559cee0f9cbc5173f3c949f6ce3525581
  Author: David Woodhouse <dwmw2@infradead.org>
  Date:   Wed Feb 6 01:36:27 2008 -0800

    Allow auto-destruction of loop devices

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>