Remove "DPLA Mapped Field" from Job and Record

[ghukill]

The idea behind mapping fields to DPLA fields has fizzled as things evolved.

However, there still needs to be a way to select what mapped field aligns with the dpla_isShownAt DPLA field. This provides a pretty reliable link for testing with the DPLA API.

Perhaps this could be integrated into XML2kvp?

• it would be on users to shunt/copy fields to a reserved field, perhaps called dpla_isShownAt to mirror what it matches against
• the beauty of this, would be removing those mapping columns entirely AND the ability to copy values from multiple places to this field (via XML2kvp)

This would remove an entire class of mappings, including a whole SQL model, which is always nice.

[ghukill]

Remove DPLAJobMap, models.py @ 2169.

[ghukill]

Done.