test: begin black-box cleanup of the e2e suite

[p-targowicz]

Deduplication pass from the cross-repo test taxonomy: the integration
repository keeps black-box, real-device e2e coverage; API/CLI/internal
behavior is owned by mender-server's test layers (per-service acceptance
suites and the integration:ng Go runner - see
mendersoftware/mender-server#2034).

Changes:

• Remove test_os_ent_migration.py. The DEFAULT_TENANT_TOKEN mechanism it
  exercises is not compiled into the open-source server; the enterprise
  server both implements it and already tests it
  (TestDefaultTenantTokenEnterprise in its integration suite, running in
  its CI). The remaining assertion, old users receiving 401 after
  migration, is a byproduct of enterprise login scoping rather than
  migration logic.
• Remove the OS-mode test_ok_preauth_and_remove and
  test_fail_preauth_existing: pure device-auth API contracts, ported to
  the integration:ng suite with per-test CI runs. The enterprise
  variants remain until equivalent enterprise-mode backend coverage
  exists; the preauth-and-bootstrap flow (which needs a real device)
  stays here.
• Mark the remaining internal-API usages as fixtures, not assertions:
  test_autogenerate_delta's tenant delta configuration (no public
  endpoint exists) and testutils' create_org (public org creation only
  exists via the signup flow). Setup may stay white-box; the behavior
  under test must not.

Net test collection: 195 -> 191, no collection errors.

[mender-test-bot]

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