Docs: remove `USE_TESTING_BUILD_IMAGE`
We don't really want people using this flag anymore. They should be using
`build.os: ubuntu-20.04` with `apt_packages` instead.
@@ -32,8 +32,6 @@ The ``DONT_SHALLOW_CLONE`` flag is useful if your code accesses old commits duri
e.g. python-reno release notes manager is known to do that
(error message line would probably include one of old Git commit id's).
-``USE_TESTING_BUILD_IMAGE``: :featureflags:`USE_TESTING_BUILD_IMAGE`
-
``LIST_PACKAGES_INSTALLED_ENV``: :featureflags:`LIST_PACKAGES_INSTALLED_ENV`
``DONT_CREATE_INDEX``: :featureflags:`DONT_CREATE_INDEX`
