package_tests: remove validation tests

[axw]

Motivation/summary

These tests are redundant with the new modeldecoder code, where we test the generated validation logic directly.

Checklist

• I have signed the Contributor License Agreement.
  - [ ] I have updated CHANGELOG.asciidoc

I have considered changes for:
- [ ] documentation
- [ ] logging (add log lines, choose appropriate log selector, etc.)
- [ ] metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. Kibana#44001)

• automated tests (add tests for the code changes, all unit tests pass locally)
  - [ ] telemetry
  - [ ] Elasticsearch Service (https://cloud.elastic.co)
  - [ ] Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
  - [ ] Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

How to test these changes

make test

Related issues

#4465

[apmmachine]

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Build stats

• Build Cause: Pull request #4467 opened

• Start Time: 2020-11-30T14:04:18.344+0000

• Duration: 41 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4739
Skipped	143
Total	4882

Steps errors

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Compress

• Took 0 min 0 sec . View more details on here
• Description: tar --exclude=coverage-files.tgz -czf coverage-files.tgz coverage

Compress

• Took 0 min 0 sec . View more details on here
• Description: tar --exclude=system-tests-linux-files.tgz -czf system-tests-linux-files.tgz system-tests

Test Sync

• Took 3 min 10 sec . View more details on here
• Description: ./.ci/scripts/sync.sh