Remove duplicate method implementation that exists on inherited class

Found this method while trying to understand how uploads work. It
looks like it's a clear duplicate of the same code that exists in the
`Row::attachments_from_columns` implementation[1].

[1] https://github.com/alphagov/whitehall/blob/7db851b66dcc6a144443ee281c5c03ae32d54253/lib/whitehall/uploader/row.rb#L136-141
Merge pull request #1801 from alphagov/remove-duplicate-attachments_from_columns-implementation

Remove duplicate method implementation that exists on inherited class