Currently there is no options to bulk delete groups available under Organization:

https://admin.atlassian.com/o/xxxxxx-xxxxx-xxxxx-xxxx-xxxxxxxxx/groups

To delete a group, one needs to open each group individually and delete it from three dots (***) menu.

Workaround

Admins can use Org API:
https://developer.atlassian.com/cloud/admin/organization/rest/api-group-directory/#api-v1-orgs-orgid-directory-groups-groupid-delete

To do this at scale, follow the workaround documented at
Delete groups from Atlassian Cloud using Postman and REST APIs or here

Users can retrieve group ids in bulk using this export tool and being sure to choose "Group Membership" and "Pivot to column"