This is how you can delete Account Groups: 

'Settings' > 'Personalization' > 'Account List' > 'Account Grouping' > 'swipe left on the group you want to delete'

or

'Accounts' > 'select and hold on the account cell' > 'remove account via drag&drop from the group'

Accounts, that do not belong to any Group are shown on top of the account list under 'My Accounts'.