Remove deprecated coredns fields for 8.0

[efd6]

What does this PR do?

This removes the non-ECS fields from the coredns module.

Why is it important?

Listed in #19039.

Checklist

• My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• Check expected test output sanity.

How to test this PR locally

TESTING_FILEBEAT_MODULES=coredns MODULES_PATH=module mage -v pythonIntegTest

Related issues

• Relates [SIEM] Remove deprecated code from 8.0 #19039

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Build stats

• Start Time: 2021-09-29T23:29:19.673+0000

• Duration: 89 min 4 sec

• Commit: 7c2d1ac

Test stats 🧪

Test	Results
Failed	0
Passed	15301
Skipped	2318
Total	17619

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[andrewkroh]

LGTM