[FIX][IMP] project: remove message_last_post groupby in task

This filter groups tasks based on a message_last_post field that is not
updated anymore since quite a long time. It has been implemented for some
internal custom use and not supported since.

As this field has no use and lead to wrong result it has to be removed. Its
purpose can still be achieved in custom using server actions activating the
message_last_post context key and adding the field in custom views.

Done after feedback from internal migration.

addons/project/views/project_views.xml

@@ -53,8 +53,6 @@
                         <filter string="Assigned to" name="User" context="{'group_by':'user_id'}"/>
                         <filter string="Stage" name="Stage" context="{'group_by':'stage_id'}"/>
                         <filter string="Company" context="{'group_by':'company_id'}" groups="base.group_multi_company"/>
-                        <separator/>
-                        <filter string="Last Message" name="group_message_last_post" context="{'group_by':'message_last_post:week'}"/>
                     </group>
                 </search>
             </field>