Consider removing next_sync_at attribute from licenses

Problem

In this comment a question around the next_sync_at attribute in licenses came up. After some evaluation, it was confirmed that this attribute isn't actually used. There's only the setting of it 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 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 as well as on the CDot side.

Result

Remove unused attribute.