Feature flag: remove unused ones (#10423)
* Feature flag: remove `USE_SPHINX_BUILDERS`
We can safely remove this feature flag since it's marked as `default_true=True`
and `future_default_true=True` in both platforms. This means that it applies to
all the projects.
* Feature flag: remove `SPHINX_PARALLEL`
It was a failed attempt to make Sphinx to run fast.
* Feature flag: remove `CLEAN_AFTER_BUILD`
It became a Django setting and it's integrated in the application now.
* Feature flag: remove `CANCEL_OLD_BUILDS`
It's enabled by default to all the projects and it has been working fine.
* Feature flag: remove `LIST_PACKAGES_INSTALLED_ENV`
This was an attempt to show debugging information. This can be achieved now with
`build.jobs` in case the user want to display this data.
Internally, we are storing some of this data in BuildData from `telemetry`
database.
* Feature flag: remove `VCS_REMOTE_LISTING`
We have been using this by default in all projects.
* Feature flag: remove `USE_RCLONE`
We have been using this for a long time and we are happy.
* Feature flag: remove `CACHED_ENVIRONMENT`
We removed this code completely and we are not doing this anymore.
* Feature flag: remove `ALLOW_DEPRECATED_WEBHOOKS`
It's not used. Only leftovers.
* Feature flag: remove `USE_TESTING_BUILD_IMAGE`
Only used by 1 projects. Its last build was 2 years ago.
* Feature flag: remove `LIMIT_CONCURRENT_BUILDS`
We have been using this by default to everybody for months now and it's working
fine. It's safe to remove.
* Test: replace `storage.sync_directory` with `storage.rclone_sync_directory`
* Remove leftovers from deprecated webhooks
* Test: check for the original builder
* Test: fix them all (hopefully)
* Lint
* More linting
* Test: mock required function
