🔥(backend) remove CollaborationService and can-edit endpoint

The CollaborationService was doing nothing since we started the
migration to yhub, all the code using it is now removed. Also the
`can-edit` endpoint and all the safeguard mechanism relying on the
presence of other users connected to the websocket will not be used
anymore, it will be possible to replace all of this with yhub, so all
this code is also removed.

src/backend/core/api/serializers.py

@@ -180,7 +180,6 @@ class DocumentLightSerializer(serializers.ModelSerializer):
 class DocumentSerializer(ListDocumentSerializer):
     """Serialize documents with all fields for display in detail views."""
 
-    websocket = serializers.BooleanField(required=False, write_only=True)
     file = serializers.FileField(
         required=False, write_only=True, allow_null=True, max_length=255
     )
@@ -210,7 +209,6 @@ class DocumentSerializer(ListDocumentSerializer):
             "title",
             "updated_at",
             "user_role",
-            "websocket",
         ]
         read_only_fields = [
             "id",
@@ -312,7 +310,6 @@ class DocumentContentSerializer(serializers.Serializer):
     """Serializer for updating only the raw content of a document stored in S3."""
 
     content = serializers.CharField(required=True)
-    websocket = serializers.BooleanField(required=False)
 
     def validate_content(self, value):
         """Validate the content field."""