Drop the legacy tags field from artefacts

Some artefacts contain a field called "tags", containing a string of 
comma-separated keywords. This field has been long deprecated, and it 
should be dropped.

This does not affect how tags are stored for artefacts, as the tag_ids 
array is in use for this.
Merge pull request #120 from alphagov/drop-legacy-tags-field

Drop the legacy tags field from artefacts