Fix plugin manager test that fail to fetch dependency

[kaisecheng]

Release notes

[rn:skip]

What does this PR do?

tests from #13218 are flickering
remove the untouchable source that fail to provide gem dependency

Why is it important/What is the impact to the user?

Gem.sources is a static setting across the project. As the new test cases fetch the dependencies from internet, a fake/ mock sources fail to provide the information, thus fail the tests.

The jenkins tests failed [7.x] [7.15]

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 (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

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[kares]

lgtm,

the CI failure looks intermittent (and is not related to the changes here)

rspec ./specs/secret_store_spec.rb:58 # Test that Logstash expands secret store variables from config