The resinApiEndpoint config option existed for legacy reasons, where the
apiEndpoint was passed in via env vars, but this is no longer the case,
and the current supervisor wouldn't run on these older versions of
resinOS anymore anyway, so I've removed the references to this legacy
endpoint, as it made reasoning about offline mode weird.
Change-type: minor
Signed-off-by: Cameron Diver <cameron@resin.io>