[FIX] account/sale: remove unused Qweb variable

That variable seems a remaining piece of old code that now doesn't
have any use and makes to compute twice the tax table each time
you print the report.

Closes #20569

addons/account/views/report_invoice.xml

@@ -109,7 +109,6 @@
                                             <span t-field="o.amount_untaxed" t-options='{"widget": "monetary", "display_currency": o.currency_id}'/>
                                         </td>
                                     </tr>
-                                    <t t-set="tax_groups_length" t-value="len(o._get_tax_amount_by_group())"/>
                                     <t t-foreach="o._get_tax_amount_by_group()" t-as="amount_by_group">
                                         <tr style="border-bottom:1px solid #dddddd;">
                                             <t t-if="len(o.tax_line_ids) == 1 and o.amount_untaxed == amount_by_group[2]">

addons/sale/report/sale_report_templates.xml

@@ -147,7 +147,6 @@
                                         t-options='{"widget": "monetary", "display_currency": doc.pricelist_id.currency_id}'/>
                                 </td>
                             </tr>
-                            <t t-set="tax_groups_length" t-value="len(doc._get_tax_amount_by_group())"/>
                             <t t-foreach="doc._get_tax_amount_by_group()" t-as="amount_by_group">
                                 <tr style="border-bottom:1px solid #dddddd;">
                                     <t t-if="amount_by_group[3] == 1 and doc.amount_untaxed == amount_by_group[2]">