Spree added stock movements to track the movements between stock
locations. But we got rid of stock locations and the only stock
movements we have now are just records of stock level changes.
These records were not created in all cases though and there were also
not created for variant overrides (inventory items). And since these
records aren't visible anywhere, I think it's best we remove them
altogether.
I do think that some kind of log would be useful but I don't think that
AR records like this are the best solution for that. And the
StockMovement model just added complexity to our already complex stock
level storage. The actual adjustment of the count_on_hand attribute of
the StockItem was performed in an after_create hook of the
StockMovement. Now we call it explicitely.
