[backend] drop circle CI build?

[alxndrsn]

odk-central-backend is tested in both GitHub Actions and CircleCI.

Almost all tests run in CircleCI are also run in the standard-suite workflow in GitHub Actions. The only difference seems to be that CircleCI runs two extra tests:

1. "Check for large files"
2. make check-file-headers

Benefits

Maintenance

Removing support for CircleCI would simplify maintenance of CI jobs. See e.g.:

• npm versions may differ in dev vs ci vs prod #1570
• [backend] ci: postgres versions are out of date #1571

Points of failure

Currently CircleCI can block builds due to unreliability. E.g. getodk/central-backend#1719 has been waiting ~40 minutes for a CircleCI job to run: