Make nm_setting_to_hash() and nm_setting_new_from_hash() private, and
remove the public nm_setting_update_secrets() wrapper around the
existing private _nm_setting_update_secrets().

These functions should really only be called from the corresponding
NMConnection-level methods, and in particular, with certain
compatibility properties in the future, we will need to consider the
entire connection all at once when setting properties, so it won't
make sense to serialize/deserialize a single setting in isolation.