permanently delete contact should not remove activities if connected to other contacts

see initial discussion here: http://issues.civicrm.org/jira/browse/CRM-13812

to reproduce:
1) create contact 1
2) create contact 2
3) create activity with source = contact 1 and target = contact 2
4) permanently delete contact 1

result: the activity record is completely removed, even though the activity was attached to another contact which was not deleted.

the problem is in: CRM_Activity_BAO_Activity::cleanupActivity()
according to the description of that method and the comments therein, we should not be deleting the activity record if contacts exist that are connected to the activity as a target or assignee. so this appears to be a bug, likely added during the activity_contact schema restructuring.