[IMP] uom: sql constraint is better than python one
[IMP] uom: sql constraint is better than python one

Since we got 2 constraints checking the same
thing (one python, one sql), it is better
to remove the python one, which is not
very performance friendly in case of huge
import.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo