Geo: Reset other fields when disabling resync_wiki on a disabled wiki

Follow up on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5089

Discovered in https://gitlab.slack.com/archives/C7S4KUEPN/p1521836761000016?thread_ts=1521834043.000633&cid=C7S4KUEPN

E.g. Some queries depend on *_retry_count, which needs to be set to nil when disabling resync_wiki in order for Geo::ProjectRegistry.failed_wikis to be accurate.