Remove payload option from completion suggester

[areek]

The payload option was introduced with the new completion
suggester implementation in v5, as a stop gap solution
to return additional metadata with suggestions.

Now we can return associated documents with suggestions
(#19536) through an additional fetch phase using stored
field (_source). The fetch phase ensures that we only fetch
the _source for the global top-N suggestions instead of
fetching _source of top results for each shard.

[jimczi]

LGTM for the removal.
I am concerned about the fetch phase though. It seems that we always retrieve the _id of the suggestion even if _source is disabled. This is expected since it is now the same code that retrieves the stored fields for the search and the suggestion. For the top docs the assumption is that the _id is always needed so we have to visit all the stored fields but for suggestions I am not sure this is what we want ?

[nik9000]

For the top docs the assumption is that the _id is always needed

I wish we could turn that off too.... It'd be nice not to have to go to stored fields at all sometimes. If you only want doc_values fields it is lame you still need to go get _id from stored fields.

[areek]

Thanks for the review and suggestion ;) @jimferenczi and @nik9000! I agree it would be nice if we could turn off retrieving the _id field for suggestion when _source is disabled. I will open a subsequent PR for this.