fix: use staging environment for EPR

[mdelapenya]

What does this PR do?

It removes the package registry service from the startup list when using the elastic-package provider.

Finally, we are reverting #1750, as it added a local pacakge registry to the compose provider, which is wrong for the ARM flavour of the test suite: there is no ARM Docker image for the package registry.

Why is it important?

Fixes the ARM test suite in 7.15, also adding consistency across branches about what package-registry to use.

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 run the Unit tests (make unit-test), and they are passing locally
• I have run the End-2-End tests for the suite I'm working on, and they are passing locally
• I have noticed new Go dependencies (run make notice in the proper directory)

Related issues

• Relates Use elastic-package again #1737

[mdelapenya]

Merging, as ARM test suite is green again

[elasticmachine]

💔 Build Failed

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Build stats

• Start Time: 2021-11-02T12:19:55.794+0000

• Duration: 46 min 47 sec

• Commit: 1013ffe

Test stats 🧪

Test	Results
Failed	0
Passed	234
Skipped	0
Total	234

Steps errors

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Restore files previously stashed

• Took 4 min 17 sec . View more details here
• Description: source

💚 Flaky test report

Tests succeeded.

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