Fix instance variables access to several nuget services

What does this MR do?

During the implementation of the NuGet packages support (spanning over several MRs), services and presenters have been created. At this time, I thought that it was a convention to have a attr_reader to access instance variables. During one of the last MRs, I was called out for using attr_reader and exposing the internal state of objects. Exposing those can be considered as a code smell.

I hate leaving code smells behind my work, as such, this tiny MR revisits all the NuGet services and presenters and removes the attr_reader access as it's not needed. Instance variables will be accessed by the standard way: using directly @XXX variables.

Screenshots

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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