[FIX] mrp: merge moves in pick before manufacturing

Use case:
It happens that a product is consumed in different operations.
So it needs two distinct BoM lines. Since commit [1] the stock.move
in pbm are not merged. However [1] was design for kit.
In our case we would like to have only one stock.move for all the
quantities.

The fix is not perfect because it won't work if we confirm at the
same time a move with a kit and without it. But at least it will let
people using MO without kit has the correct behavior

+ remove a duplicate of the function

[1] commit 741d2fe9

closes odoo/odoo#120835

Signed-off-by: William Henrotin (whe) <whe@odoo.com>