remove instruction and description fields from field based and metadata description exports

[saracarl]

Since the instruction and description field types don't require (or allow) input from users, it seems unnecessary for these to be exported (the label [and fieldname] can be an entire sentence or two). It's easy to just delete those columns, though, so it’s not that that important.

[benwbrum]

To reproduce this for field-based transcription,

• Create a collection
• Change the transcription type to Field-Based
• Add a field of each data type
• Import a work
• Transcribe a page of the work, filling in values for each field, and save
• Click the Export tab
• Export Tables/Fields
• Inspect the spreadsheet created for the page; note that the fields of type Instruction and Description both have columns, even though they do not contain user-created data

To reproduce this for metadata projects,

• Edit a collection
• Enable Metadata Description
• Edit metadata fields
• Add a field of each data type
• Click on a work
• Press "create metadata"
• Fill in values for each field and save
• Click the Export tab
• Export Work Metadata
• Inspect the spreadsheet created for the page; note that the fields of type Instruction have columns, even though they do not contain user-created data