wikibase: Don't show "required statement" constraints for edits on existing entities

When uploading information to existing Wikibase items, we can get "issues" such as:

male population (P1540) requires statement with property P17. This property is expected to have another statement with property P17 such as on item Katowice (Q588). If the item already has statements with property P17 in Wikidata, then this warning can be ignored.

During an OpenRefine-Wikidata training at Wikimania, this confused the trainer and audience into thinking that the "male population" property was only applicable to countries, whereas the meaning of this constraint is just to say that if "male population" is present on an item, the country of that item should also be added as a statement.

This issue could actually be safely ignored in this case because the corresponding items already had such a statement (which OpenRefine didn't check, as the issue message indicates).

Proposed solution

Such issues should simply not be generated for edits on existing items, because it's confusing.

Alternatives considered

The issue could be generated only in "slow mode" (when showing issues in the upload dialog), after checking that the corresponding statements are indeed missing.

Additional context

Follow-up to the training session mentioned above.