[FIX] reduce the number of unnecessary calls to trg_write in cancel_assign

when calling stock_picking.cancel_assign, the original code would call trg_write nb_move+1 times. This version only performs one call
[FIX] reduce the number of unnecessary calls to trg_write in cancel_assign
Merge branch 'ocb-7.0' of github.com:akretion/openerp-addons into ocb-7.0