Skip invalid entities in the batch delete instead of failing

[frankrousseau]

Problem

• deletion_service.remove_entities (batch delete-entities route, merged in Add batch action routes for multi-selection Kitsu operations #1145) raised a WrongParameterException as soon as one id did not belong to the project or was not a deletable type, aborting the whole batch.
• A selection containing a single stray entity (wrong project, or a sequence) therefore deleted nothing.

Solution

• Skip entities that do not belong to the project or are not an asset, shot, edit or concept, and return the ids actually removed.
• Update the tests accordingly (skip unsupported type, skip entity from another project) and drop the now-unused exception and the 400 response from the route doc.

Follow-up to #1145 (merged before this change landed).