Fix EZP-23152: deleteVersion removes images from published version

JIRA: https://jira.ez.no/browse/EZP-23152

Image references are removed in the assumption there are no other field ids using it, which may not be true.
This adds a check for additional versions using the same id, and does not remove the reference in that case.
Be more verbose on tests method names:

`testThatRemovingDraftDoesntRemovePublishedImages` ? :-)