If a group is deleted from the tool, its historical results will disappear.
Removing (or moving) a sub-group from its top-group is OK and will not delete any historical results. Although, it is good to keep in mind that if you remove a sub-group from a scheduled target group, the results will not be visible since the group is no longer included in the targeted scheduling. If you move a group from the group structure and the results disappear, you can simply add the group back to its original place (or somewhere else in the structure) and the results will be visible again.
To remove a sub-group from a top-group, simply open the top-group and click on the cross that appears when you hold your pointer on the group name in the section "Sub-groups" (see below picture).