Remove brittle and now unnecessary flattening of…

`file` columns in `select_one_from_file` and `select_multiple_from_file`
rows. This was causing a `KeyError` when the asset content predated

Flattening of `{"type": "select_one_from_file", "file": "choices.csv"}`
(and equivalent `select_multiple_from_file` structures) to the
XLSForm-standard `{"type": "select_one_from_file choices.csv"}` is
handled by kobotoolbox/formpack#318 and kobotoolbox/formpack#319.

kpi/models/asset_snapshot.py

@@ -214,12 +214,6 @@ def generate_xml_from_source(self,
         warnings = []
         details = {}
 
-        for row in source_copy['survey']:
-            if row.get('type') in ['select_one_from_file', 'select_multiple_from_file']:
-                ff = row.pop('file')
-                _type = row.pop('type')
-                row['type'] = f'{_type} {ff}'
-
         try:
             xml = FormPack({'content': source_copy},
                             root_node_name=root_node_name,