[FIX] website_slides: avoid putting unexisting attribute on object

In this computed field, an attribute is assigned to the record
but this attribute does not exist. Python authorized this but
this makes the code ugly and this is definitely not a good
practice.

Task-1941250

addons/website_slides/models/slide_channel.py

@@ -184,8 +184,7 @@ class Channel(models.Model):
         for cp in channel_partners:
             result.setdefault(cp.channel_id.id, []).append(cp.partner_id.id)
         for channel in self:
-            channel.valid_channel_partner_ids = result.get(channel.id, False)
-            channel.is_member = self.env.user.partner_id.id in channel.valid_channel_partner_ids if channel.valid_channel_partner_ids else False
+            channel.is_member = channel.is_member = self.env.user.partner_id.id in result.get(channel.id, [])
 
     @api.depends('slide_ids.slide_type', 'slide_ids.is_published', 'slide_ids.completion_time',
                  'slide_ids.likes', 'slide_ids.dislikes', 'slide_ids.total_views')