* get rid of CUSTOM_ATTRIBUTES. Replace it with IFullTextIndexSerializable adapter `custom_indexable_attributes`
  attribute;
* FullTextIndexSerializable.serialize method : default complete value is now True*


closes #17079905

otherwise the import just hand forever.

since it's a cube as package which are supported from there.

Besides from being a debug-only trace, it raises an error when
no actual document is indexed.

It's not used (and therefore not real-life-tested) anywhere
and current implementation is known to be partial and a bit
hard to maintain.

we should not crash if we try to delete an inexisting document

(closes #17071989)