Retire the Cirrus demo-app integration harness now that experimenter's own Cirrus integration is tested

[jaredlockhart]

#16994 added an integration test for experimenter's own Cirrus integration (#16992), which replaces the demo app as the way we exercise Cirrus end to end. The demo-app harness it superseded is still on main, and no CI job selects it.

Remaining pieces:

• experimenter/tests/integration/nimbus/test_cirrus_integration.py
• experimenter/tests/integration/nimbus/pages/demo_app/ and the demo_app fixtures in conftest.py
• the cirrus_enrollment marker in experimenter/tests/pytest.ini
• the demo-app-server / demo-app-frontend services in docker-compose-cirrus.yml and docker-compose-integration-test.yml, the Makefile target that builds them, and the top-level demo-app/ directory

Acceptance criteria:

• Everything above is removed, except whatever the test(cirrus): add integration test for experimenter's own cirrus integration #16994 test still needs (check whether it relies on the cirrus service in docker-compose-cirrus.yml or on COMPOSE_CIRRUS before removing them).
• The test(cirrus): add integration test for experimenter's own cirrus integration #16994 test and the rest of the integration suite stay green.

┆Issue is synchronized with this Jira Task