Remove Ponos from CE

Based on !2227 (merged)

TODO:

• Remove & update tests

arkindex/project/serializer_fields.py

@@ -293,8 +293,12 @@ class DatasetSetsCountField(serializers.DictField):
         return elts_count
 
 
-class NullField(serializers.Field):
-    read_only = True
+class NullField(serializers.CharField):
 
     def to_representation(self, value):
         return None
+
+    def to_internal_value(self, data):
+        if data is not None:
+            self.fail("invalid")
+        return None