[FIX] *: remove unnecessary t-key

-Since [1] we have add t-key for some template, but some aren't
necessary because they not load via Owl, therefore remove it to avoid
warning : "Unknown directives or unused attributes"

[1]: https://github.com/odoo/odoo/pull/130467/commits/a9b577dfcbd6c01abdc649a3ea61f80e9018832d#diff-0ba40f64ee7d28c08bdbb7273985adc0d2d2b63d8c3233aa2630e6ba4bde4df9

closes odoo/odoo#160863

Signed-off-by: Samuel Degueldre (sad) <sad@odoo.com>

addons/mail_group/views/portal_templates.xml

@@ -215,7 +215,7 @@
                 </a>
             </p>
             <div class="o_mg_link_content d-none row justify-content-center">
-                <div class="mx-4 my-3 text-center o_mg_attachment" t-foreach="attachments" t-as="attachment" t-key="attachment_index">
+                <div class="mx-4 my-3 text-center o_mg_attachment" t-foreach="attachments" t-as="attachment">
                     <t t-if="attachment.access_token">
                         <a t-attf-href="/web/content/#{attachment.id}?download=true&amp;access_token=#{attachment.access_token}" target="_blank">
                             <div class="oe_attachment_embedded o_image" t-att-title="attachment.name" t-att-data-mimetype="attachment.mimetype" t-attf-data-src="/web/image/#{attachment.id}/100x80?access_token=#{attachment.access_token}"/>

addons/portal_rating/views/rating_templates.xml

@@ -24,13 +24,13 @@
                 </small>
             </t>
             <t t-else="">
-                <t t-foreach="range(0, val_integer)" t-as="num" t-key="num_index">
+                <t t-foreach="range(0, val_integer)" t-as="num">
                     <i class="fa fa-star" role="img"></i>
                 </t>
                 <t t-if="val_decimal">
                     <i class="fa fa-star-half-o" role="img"></i>
                 </t>
-                <t t-foreach="range(0, empty_star)" t-as="num" t-key="num_index">
+                <t t-foreach="range(0, empty_star)" t-as="num">
                     <i class="fa fa-star-o" role="img"></i>
                 </t>
                 <small class="text-muted ms-1">