Back when we introduced the fields parameter to the search API, it could only fetch values from _source, hence
the corresponding sub-fetch phase fails early whenever _source is disabled. Today though runtime fields can
be retrieved from a separate value fetcher that reads from fielddata, and metadata fields can be retrieved
from stored fields. These two scenarios currently throw an unnecessary error whenever _source is disabled.
This commit removes the check for disabled _source, so that runtime fields and metadata fields can be retrieved even when _source is disabled. Fields that need to be loaded from _source are simply skipped whenever _source is disabled, similar to when a field is not found in _source.
Closes#87072
50793a68a8