The Sandbox Solution Server Extension has been deprecated, and is no longer required for migration. This article will explain how to deactivate it.
Go in the SharePoint site collection settings and click on Solutions.
If you can't see the feature Solutions under Web Design Galleries, it is because custom scripts are not activated on the site collection level. For information on activating custom scripts, see here.
Select the current installation of the Server Extension.
It is possible that, in your case, the solution was already deactivated. If you cannot press Deactivate, that means it has already been deactivated.
Once deactivated, you can click Delete.