core: ignore image does not exists error on remove memory state

If we are trying to remove the memory state images, and we find that the
images don't exist, we treat this as if the operation was successful
(because the images are not there anymore) instead of treat this as an
error.

Change-Id: Ic5dce38bed0675306d647b3830fe2d208e19d0d1
Signed-off-by: Arik Hadas <ahadas@redhat.com>