[FIX] mrp: BoM structure report with variants

1. Setup a few variants for a product;
2. Create a BoM for each of the variants with different
components/quantities, make sure to specify the product variant field;
3. Go to any of the variants and click Print BoM Structure.

Before this commit, it will print the BoM structure for all variants and
not only the one selected. Also, it will print in all the variants the
same BoM structure. If multiples BoM were selected to print, each
variant of the product was printed with each of the BoM structure
selected.

Now, if you select one BoM, only this is printed. If multiples BoM are
printed, each product, and product variant are correctly related to
their respective BoM structure.

opw-2007939

closes odoo/odoo#34353

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>

addons/mrp/report/mrp_report_bom_structure.py

@@ -14,8 +14,7 @@ class ReportBomStructure(models.AbstractModel):
         docs = []
         for bom_id in docids:
             bom = self.env['mrp.bom'].browse(bom_id)
-            variant = data and data.get('variant')
-            candidates = variant and self.env['product.product'].browse(variant) or bom.product_tmpl_id.product_variant_ids
+            candidates = bom.product_id or bom.product_tmpl_id.product_variant_ids
             for product_variant_id in candidates:
                 if data and data.get('childs'):
                     doc = self._get_pdf_line(bom_id, product_id=product_variant_id, qty=float(data.get('quantity')), child_bom_ids=json.loads(data.get('childs')))