[FIX] mrp: Delivered quantity with kit

Steps to reproduce:

- Set the Decimal Accuracy at 5 digits for Product UoM
- Create a product X
- Create a kit BOM for X with a component Y, and qty 0.08600
- Create a Sale Order for 10 product X, confirm and deliver all

Issue

- On the SO, quantity delevered is 9.00000 instead of 10.00000, same
  issue occur with the same flow with a Purchase Order.

Cause

As the quantity per kit was rounded when calling _compute_qty,
the calcul of quantity ratio was not well computed.

Solution

Avoid rounding the quantity per kit, as the quantity ratio is rounded
a few steps after.

opw-2590126

closes odoo/odoo#78400

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>