Remove embeds of unmappable properties

[drio18]

Here, we remove embeds of properties that cannot be mapped within our system, namely those that fall under additionalProperties or patternProperties in our schema.

As far as I understand things, since these fields cannot be mapped, adding them to an item's embedding list will not work regardless of the changes here, specifically the explicit removal of the properties from the default embeds in find_default_embeds_for_schema. Thus, no properties in the schema defined under additionalProperties or patternProperties can be embedded or used for invalidation scope with our current set-up, and significant refactoring would be required to make these work.

[willronchetti]

Looks good, should be tested with FF/CGAP to be certain but otherwise should be fine to merge