As part of adding support for new API to the high-level REST client,
we added support for the `flat_settings` parameter to some of our
request classes. We added documentation that such flag is only ever
read by the high-level REST client, but the truth is that it doesn't
do anything given that settings are always parsed back into a `Settings`
object, no matter whether they are returned in a flat format or not.
It was a mistake to add support for this flag in the context of the
high-level REST client, hence this commit removes it.
Luca Cavanna