[IMP] theme_*: review s_picture snippet occurrences

The goal of this commit is to fix xpath after the `s_picture` snippet
redesign, and eventually tweak some classes.

task-4047748

closes odoo/design-themes#824

Related: odoo/odoo#172740
Signed-off-by: Arthur Detroux (ard) <ard@odoo.com>

theme_artists/views/snippets/s_picture.xml

@@ -4,11 +4,10 @@
 <template id="s_picture" inherit_id="website.s_picture">
     <!-- Section -->
     <xpath expr="//section" position="attributes">
-        <attribute name="class" add="o_cc5" remove="o_cc2" separator=" "/>
+        <attribute name="class" add="o_cc5" separator=" "/>
     </xpath>
     <!-- Image -->
     <xpath expr="//img" position="attributes">
-        <attribute name="class" remove="img-thumbnail" separator=" "/>
         <attribute name="style">width: 100%;</attribute>
     </xpath>
 </template>