[IMP] calendar: removes the group-by availability preset filter

It turns out that the group-by 'availability' filter of the
'calendar.event' model is not really meaningful. We will hence remove it
to simplify the interface. Note that the users will still be able to group
the records by 'availability' by defining a custom group.

task-2646322

closes odoo/odoo#76901

Related: odoo/enterprise#21038
Signed-off-by: awa-odoo <awa-odoo@users.noreply.github.com>

addons/calendar/views/calendar_views.xml

@@ -317,7 +317,6 @@
                 <filter string="Archived" name="inactive" domain="[('active', '=', False)]"/>
                 <group expand="0" string="Group By">
                     <filter string="Responsible" name="responsible" domain="[]" context="{'group_by': 'user_id'}"/>
-                    <filter string="Availability" name="availability" domain="[]" context="{'group_by': 'show_as'}"/>
                 </group>
             </search>
         </field>