Mailmotor: fix bug when deleting multiple groups

Deleting mulitple groups deletes the groups correctly in CampaignMonitor, but only deletes the first group in Fork.
After that you're unable to delete the remaining Fork groups, since they don't match an existing group in CM.
The fix deletes the group anyway if it doesn't exist anymore in CM.
Merge pull request #634 from Langeuh/cm-list-bug

Mailmotor: fix bug when deleting multiple groups