[FIX] mrp: fix uom conversion in bom kit

no need to make rounding during computing qty_available

STEPS (see the test):

* create product with uom dozens
* create product with uom units
* create bom kit to convert one to another
* set qty on hand to 1 for product dozens
* check qty for product units

BEFORE: qty=11
AFTER:  qty=12

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opw-2632782

closes odoo/odoo#76145

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>