refactor: Remove defunct squirrel girl beta header

[TrishGillett]

The squirrel girl preview header made the github API provide additional fields, primarily around reactions, for a few endpoints.

However, I suspect it's not doing anything anymore:

• We're seeing reactions cols populated for IssuesStream, but they are consistently null for PullRequestsStream.
• The reactions cols are not included in the example responses for either endpoint (issues, pull requests)
• I found this announcement which isn't as explicit a smoking gun as I would like, but in theory should cover the squirrel girl preview since it existed at the time the announcement was published. ie. either squirrel girl was one of the 23 previews that got promoted, or it was one of the 2 that were intended to be sunset by end of year 2021.
• My hypothesis is that they did graduate the squirrel girl preview, but only for the issues endpoint.

I'll run some tests like this and confirm that results for both streams are the same whether or not the header is used.

Also to be determined:

• Should we remove the sunsetted columns from the schema, or keep them there and let them continue to be null to avoid causing disruptions for users?
• The integer column comments is also unpopulated by the pull requests endpoint in practice and in the example response. It's not clear that it is related to the squirrel girl preview at all, but should it be cleaned up too?

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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