Remove stale test suites and update some flags.

[ScottTodd]

• Delete iree_special_models (migrated to https://github.com/iree-org/iree/tree/main/experimental/regression_suite/shark-test-suite-models, outdated and failing in this repo: https://github.com/nod-ai/SHARK-TestSuite/actions/workflows/test_iree.yml)
• Delete iree_tests/benchmarks/sdxl (same as previous item)
• Delete turbine_tank (unmaintained since https://github.com/nod-ai/SHARK-TestSuite/pull/78, not running on CI anywhere I can see)
• Update XFAILs so test_iree.yml passes again