ci: Remove "default" test env

It is unclear what the point is of maintaining a "default", the meaning
of which is unclear.

ci/test/00_setup_env.sh

@@ -46,8 +46,6 @@ export TEST_RUNNER_TIMEOUT_FACTOR=${TEST_RUNNER_TIMEOUT_FACTOR:-40}
 export TEST_RUNNER_ENV=${TEST_RUNNER_ENV:-}
 export RUN_FUZZ_TESTS=${RUN_FUZZ_TESTS:-false}
 
-export CONTAINER_NAME=${CONTAINER_NAME:-ci_unnamed}
-export CI_IMAGE_NAME_TAG=${CI_IMAGE_NAME_TAG:-ubuntu:20.04}
 # Randomize test order.
 # See https://www.boost.org/doc/libs/1_71_0/libs/test/doc/html/boost_test/utf_reference/rt_param_reference/random.html
 export BOOST_TEST_RANDOM=${BOOST_TEST_RANDOM:-1}