nm_setting_ip_config_next_valid_dns_option() API was added in libnm 1.2, but
it was never exported in the ABI of libnm. It thus was unusable, and any user
trying to link against it would have been unable to do so.

Hide the API now entirely. It doesn't seem a very nice API. If we want to
allow the user to validate option names, we should expose such a function
to validate an option (not to fetch the next valid option from a
profile).

Fixes: 019943bb ('libnm-core: add dns-options property to NMSettingIPConfig')
(cherry picked from commit e8e5c124)
(cherry picked from commit 04946f71)