Remove some dead code

[henrymercer]

The Action requires GHES 3.3, so we can safely remove this fallback logic for GHES 2.22 and GHES 3.0.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[aeisenberg]

Looks like GITHUB_ACTION_REPOSITORY is not being set during the integration tests. See:

https://github.com/github/codeql-action/actions/runs/3532744788/jobs/5927496032#step:4:14

[aeisenberg]

Otherwise looks reasonable.

[henrymercer]

So GITHUB_ACTION_REPOSITORY is only set when calling an Action from another repository. We still need to populate it manually if we're running locally.

I've updated the PR to only remove the code that deals with the fact that GHES 2.22 didn't populate GITHUB_ACTION_REPOSITORY even when the Action is called from anothe repository.

[aeisenberg]

LGTM assuming all jobs pass.