[FIX] document_page_approval: Delete diff in email_template to preven…

…t errors in some cases.

If we added image in base64 inside content:
<img src="data:image/png;base64,xxxxxx" />
when Odoo try to render message and send try to create ir.attachment with type=content (it's not possible).

document_page_approval/data/email_template.xml

@@ -29,11 +29,6 @@
 <p>${object.summary}</p>
 % endif
 
-<h3>Diff</h3>
-<div style="overflow-x:scroll; font-size:0.85em; margin-bottom:2em;">
-${object.diff|safe}
-</div>
-
 <p>Have a great day.</p>
 
 --<br/>