API support for flexible ActivityContact deletes

If we have an ActivityContact API, we can expose an API method to delete "assignee for activity ID 123", rather than "update Activity 123 and clear the assignee" (via deleteActivityAssignee parameter).

This involves making the ActivityContact API delete with alternate parameters, eg

civicrm_api3('ActivityContact', 'delete', array('activity_id' => 123));
civicrm_api3('ActivityContact', 'delete', array('activity_id' => 123, 'contact_id' => 123));
civicrm_api3('ActivityContact', 'delete', array('activity_id' => 123, 'contact_id' => 12, 'record_type_id' => 1));