ON COMMIT actions and inheritance

Hi,

Michael pointed out a problem with specifying different ON COMMIT actions
on a temporary inheritance parent and its children:

https://www.postgresql.org/message-id/20181102051804.GV1727%40paquier.xyz

The problem is that when PreCommit_on_commit_actions() executes an ON
COMMIT DROP action on the parent, it will drop its children as well.  It
doesn't however remove the children's own actions, especially ON COMMIT
DELETE ROWS, from the list and when it gets around to executing them, the
children are already gone.  That causes the heap_open in heap_truncate to
fail with an error like this:

ERROR:  XX000: could not open relation with OID 16420
LOCATION:  relation_open, heapam.c:1138

One way to fix that is to remove the tables that no longer exist from the
list that's passed to heap_truncate(), which the attached patch implements.

Thanks,
Amit