Zap dead code

[hellais]

As part of this PR, everything that is dead code, is removed from the source tree.

This includes:

• af/ -> this was the old airflow postgresql related code
• old_backend/ -> this was the super old pre-2016 twisted based backend
• docs/ -> contained documentation about the ~2016 data pipeline
• Readme.md -> Still talks about some super old stuff
• rate_limit_quotas.py -> duplicate of file inside ooniapi/rate_limit_quotas.py
• ooniapi/scripts -> these scripts expect a postgresql database, which doesn't exist anymore
• api/refresh_deps -> non-existent files are referenced in it
• markdown/* -> these docs are not used anywhere and are superseeded by the swagger based docs

[hellais]

It looks like the debops CI tool is not happy that af/prio got deleted, since it's trying to build it:

Traceback (most recent call last):
  File "/__w/backend/backend/./debops-ci", line 5[9](https://github.com/ooni/backend/actions/runs/7752829228/job/21142995136?pr=798#step:8:10)8, in <module>
debops-ci version 2023-11-22.2
    main()
  File "/__w/backend/backend/./debops-ci", line 581, in main
    ci(args)
  File "/__w/backend/backend/./debops-ci", line 297, in ci
Running apt-get update -q
KW: {}
Running apt-get install -qy --no-install-recommends git
KW: {}
Running git merge-base remotes/origin/master HEAD
KW: {}
Running git diff --name-only 48a4634cc3ef93e670c8aae4ac5830[11](https://github.com/ooni/backend/actions/runs/7752829228/job/21142995136?pr=798#step:8:12)8e3c56ee
KW: {}
Building [PosixPath('af/prio'), PosixPath('analysis'), PosixPath('api')]
Running apt-get install -qy --no-install-recommends devscripts
KW: {}
Building package in af/prio
Running dpkg-parsechangelog --show-field Version
KW: {'cwd': PosixPath('af/prio')}
    pkgs_lists = [buildpkg(pd) for pd in pkgdirs]
  File "/__w/backend/backend/./debops-ci", line 297, in <listcomp>
    pkgs_lists = [buildpkg(pd) for pd in pkgdirs]
  File "/__w/backend/backend/./debops-ci", line 209, in buildpkg
    ver = run2("dpkg-parsechangelog --show-field Version", cwd=p)
  File "/__w/backend/backend/./debops-ci", line 80, in run2
    p = run(cmd.split(), capture_output=True, **kw)
  File "/usr/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.9/subprocess.py", line [18](https://github.com/ooni/backend/actions/runs/7752829228/job/21142995136?pr=798#step:8:19)[23](https://github.com/ooni/backend/actions/runs/7752829228/job/21142995136?pr=798#step:8:24), in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: PosixPath('af/prio')

We should figure out how to teach it that we don't need that package anymore.

[hellais]

I have a patch for this inside of ooni/sysadmin#511

[hellais]

Ok we are all green, except for the integration tests, which are failing because of the changes to the public list measurements API that are part of the parent PR.

I plan to fix those tests as part of that separate PR.

This branch is ready for review.