[REFACTOR][TIR] Remove IRTransform in favour of tvm_ffi.structural_map

[tqchen]

Replace the legacy TIRx IRTransform API with the typed tvm_ffi.structural_map interface while preserving the existing postorder replacement behavior.

• Remove the C++ declaration, implementation, registration, and Python wrapper for IRTransform.
• Migrate all remaining postorder-only tile-primitive tests to typed structural_map callbacks.
• Remove the dedicated mixed preorder/postorder legacy API test; no production caller uses both phases in one pass.
• Validate the changed area with 128 passing tests and compare the full-suite failure set against the parent tree.