Simplify export file format after JSONB migration is performed

[giovannipizzi]

Currently there is a convoluted way of storing datetimes in the export file, to mark them as date times and not as strings (the attributes dictionary is "duplicated", having None values in every place that maps a standard data type, except for value related a date time, where a special string is used). This is the node_attributes_conversion field, see docs

If in #2788 we remove support for datetimes, we can safely remove this dictionary from the data.json - this will reduce the export file size, simplify the code and most probably also make import/export faster.

[CasperWA]

We need to make sure that node_attributes_conversion is used only for datetime objects.

When removing node_attributes_conversion, the newly added node_extras_conversion dict can also be removed from the export files.