[FIX] mrp: blocks default product on bom line

The action called from the product's bom stat button (mrp.product_open_bom)
fill the key 'default_product_id'. This allows, creating a bom from the list view,
to prefilled the product.

This context key will also fill each new bom line as the technical field is
called product_id too.

This commit overrides default_product_id to False for the bom line One2many
in the bom form view in order to only prefill the finished product and not
the components.

Task : 2045485

closes odoo/odoo#35444

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>