EZP-21324: image files can be removed by mistake

Fixes http://jira.ez.no/browse/EZP-21324, replaces #792.

Since copies of archived versions aren't processed by `eZDatatype::onPublish()`, they still reference the same images than the original.

The `eZImageAliasHandler::removeAliases()` method didn't take this into account, and could delete images that were actually used by the original content.

This patch prevents the method from deleting images that don't have an entry in the ezimagefile table, since they are not created during copy, except for the version that gets republished.

## TODO
- [ ] Cherry-pick tests from #792