opal_check_portals.m4: remove this unused file

It looks like OPAL_CHECK_PORTALS is not used anywhere; it looks like
it has been completely replaced by opal_check_portals4.m4
(OPAL_CHECK_PORTALS4).
Merge pull request #274 from jsquyres/topic/delete-opal-check-portals.m4

opal_check_portals.m4: remove this unused file