Delete unused repository secrets and variables

[dbeatty10]

We have repo secrets and variables that I believe are not in use; we should delete them to avoid confusion.

c.f. #378

As mentioned in #390, we have two different workflows:

1. ci.yml
2. integration_tests.yml

The first uses both vars and secrets.

Whereas the second looks like it is (mostly) using secrets.

[github-actions]

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