For that you could just find and edit that kanban view in order to remove this attr: on_create="crm.create_opportunity_simplified"

You could do it easy with developer mode activated by editing the kanban view in that menu.

If you wanna do it in a module with xml view inheritance, you could do it like:

<record id="crm_case_kanban_view_leads" model="ir.ui.view">
    <field name="name">crm.lead.kanban.lead</field>
    <field name="model">crm.lead</field>
    <field name="inherit_id" ref="crm.crm_case_kanban_view_leads"/>
    <field name="arch" type="xml">
        <xpath expr="//kanban" position="attributes">
            <attribute name="on_create"/>
        </xpath>
    </field>
</record>