Removing Module Bays with Children Power Ports breaks Power Ports count

[Breadberry07]

NetBox version

v3.6.4

Python version

3.8

Steps to Reproduce

1. Create module types for device (for example PWR-MX960-4100-AC with 2 power ports):
   
2. In Devices add to chassis device (for example mx960) PEM module:
   
3. Check count Power Ports:
   
4. Delete module from device:
   

Expected Behavior

For the Power Ports count to be correct

Observed Behavior

The port counter has not decreased

[kkthxbye-code]

Similarly removing a rearport that is associated with a frontport will not decrement the rearport count. Deleting an inventory item that has children also does not account for the children in the count, so deleting an inventoryitem named parent_inventoryitem that has one child inventory item child_inventoryitem will decrese the counter by 1 instead of 2.

I assume the counting logic is broken for any and all CASCADE deleted objects.