Refactor check for import_id_fields

[matthewhegarty]

Problem

Closes #1644

• The check for invalid import_id_fields did not need to be performed for every row

Solution

• Replaced with single check before import

Acceptance Criteria

• Addition of tests
• Removed unnecessary test

[pokken-magic]

Nothing really blocking here, seems like an overall reasonable thing to do, but some tiny thoughts to consider.