Remove contact field from interactions in main API

[reupen]

Description of change

This removes the deprecated contact field from /v3/interaction and /v3/interaction/<id>.

Checklist

• Has a new newsfragment been created? Check changelog/README.rst for instructions
• Have any relevant search models been updated?
• Have any relevant fixtures (fixtures/test_data.yaml) been updated?
• Have any relevant select-/prefetch-related field lists in the views and search apps been updated?
• Has the admin site been updated (for new models, fields etc.)?
• Has the README been updated (if needed)?

[codecov]

Codecov Report

Merging #1501 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1501   +/-   ##
========================================
  Coverage    96.91%   96.91%           
========================================
  Files          267      267           
  Lines         7855     7855           
  Branches       757      757           
========================================
  Hits          7613     7613           
  Misses         164      164           
  Partials        78       78

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