🔥(pagination) remove unused ordering field

Our Pagination class inherits from the PageNumberPagination Django class.
However, this base class as not ordering attribute. Thus, setting a
default value wont have any effect on the code.

Why did we end up passing a value to this non-existing attribute? Becasue
we copy/pasted some code sources from Joanie, and Joanie also has this
attribute set to a default value.

If you take a look at DRF pagination style documentation, the only three
attributes they set on the child class are 'page_size', 'max_page_size'
'page_size_query_param'. 'ordering' is not mentionned in the attributes
you may override. However, the CursorPagination class offers the latter
attribute, which may explain why we did end up setting this non-existing
attribute in Joanie.

src/backend/core/api/viewsets.py

@@ -97,7 +97,6 @@ class SerializerPerActionMixin:
 class Pagination(pagination.PageNumberPagination):
     """Pagination to display no more than 100 objects per page sorted by creation date."""
 
-    ordering = "-created_on"
     max_page_size = 100
     page_size_query_param = "page_size"