ci: Remove CircleCI

[alexec]

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed the CLA.
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[alexec]

Merge as soon as approved.

[simster7]

@alexec I think you might have to remove Circle from the repository in some other way. The reason I kept the dummy job was because even if the CI config file was removed, it would still expect something to run

[alexec]

I've deleted us from CircleCI via their settings page so it will stop complaining now and this PR will be OK.