Consider removing next_sync_at attribute from licenses
### Problem In [this comment](https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/incident-management/-/issues/768#note_2395296604) a question around the `next_sync_at` attribute in licenses came up. After [some evaluation](https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/incident-management/-/issues/768#note_2395791776), it was confirmed that this attribute isn't actually used. There's only the [setting of it](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/f857fb5c59d253d4fb4b3f222552e93d94bed9d1/app/models/license.rb#L320) on the CDot side but nothing on the GL side. The only reference to the info around this attributes was found in https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/1076+. It confirms that this attribute was never fully implemented and isn't really needed since we have the daily syncs in place. ### Proposal Consider removing the `next_sync_at` attribute. Removing this attribute will not result in the generation of new licenses for customers due to the attribute being [ignored](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/2d2b50ecbcf74abae8f16a432961fcebd6c106b5/app/models/encrypted/license.rb#L25) for the license comparison check to determine if a new license is needed. The removal includes any mentions of it in the [`gitlab-license` gem](https://dev.gitlab.org/gitlab/gitlab-license) as well as on the CDot side. ### Result Remove unused attribute.
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