[7.0] stock: call to stock.picking cancel_assign causes many unnecessary calls to workflow trg_write

[Alexandre Fayolle - camptocamp]

When calling stock_picking.cancel_assign, a first call to trg_write for the picking is done, and then the stock_move.cancel_assign is called, which will cause a call to trg_write of the picking for each move. This is not efficient at all.

The correct way of doing things is to rely on stock_move.cancel_assign to call trg_write for the picking and to make sure that trg_write is called once for each different picking related to the list of moves (in case the stock_move.cancel_assign is not called by stock_picking).