[Elastic Agent] Remove symlink.prev from previously failed upgrade

[michalpristas]

What does this PR do?

This PR removes symlink prev before starting upgrade procedure

Why is it important?

In case .prev is left there because of failure in previous upgrades it will fail to upgrade on cannot create symlink, file already exists error

Fixes: #26513

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.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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• Start Time: 2021-07-08T07:42:02.240+0000

• Duration: 70 min 15 sec

• Commit: 7d6f513

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Test	Results
Failed	0
Passed	6972
Skipped	16
Total	6988

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Test stats 🧪

Test	Results
Failed	0
Passed	6972
Skipped	16
Total	6988

[blakerouse]

Looks good.