[FIX] mrp_byproduct: remove bom uos when calculating byproduct

In commit 43977deb a mrp.bom was transformed from a recursive model
which may contains BoMs containing BoMs and so on, to a simpler model
containing a line of products (mrp.bom containing mrp.bom.line).

mrp_byproduct wasn't changed to reflect on this change which caused an
error.

Since there is no product Unit of Sale on a byproduct, the current
behaviour causing the error of trying to multiply based on the Unit of
Sale of the BoM product has been removed.

closes #7806
opw-645639
opw-645640