flows: remove stage challenge type #22204

Job	Run time
lint (bandit)	2m 4s
test-e2e (proxy)	4m 54s
test-integration	5m 17s
lint (black)	2m 6s
test-e2e (oauth)	7m 51s
test-e2e (oauth-oidc)	5m 45s
lint (codespell)	1m 48s
test-migrations	3m 14s
test-e2e (saml)	6m 45s
lint (pending-migrations)	1m 55s
test-migrations-from-stable - PostgreSQL 15-alpine	16m 19s
test-e2e (ldap)	8m 45s
lint (ruff)	1m 50s
test-migrations-from-stable - PostgreSQL 16-alpine	15m 47s
test-e2e (radius)	4m 42s
test-unittest - PostgreSQL 15-alpine	14m 58s
test-e2e (scim)	3m 59s
test-unittest - PostgreSQL 16-alpine	15m 3s
test-e2e (flows)	6m 2s
ci-core-mark	1s
build (amd64)	9m 18s
build (arm64)	18m 47s
pr-comment	5s
	2h 37m 15s

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