[Refactor][Meta-schedule] Remove meta-schedule as_string mechanism in favor of default representation

[tlopex]

Python-side meta-schedule classes (PyCostModel, PyFeatureExtractor, PyMeasureCallback, PyScheduleRule, PyMutator, PyPostproc) carried an f_as_string callback whose only purpose was to produce a repr-style string (s_tir.meta_schedule.<SubclassName>(0x...)) for str(...).

This mechanism stopped working after #19461 migrated ReprPrinter to the tvm-ffi __ffi_repr__ mechanism and intentionally removed the per-type set_dispatch<Py*Node> hooks that called back into f_as_string, which broke three *_as_string tests:

• test_meta_schedule_cost_model.py::test_meta_schedule_cost_model_as_string
• test_meta_schedule_feature_extractor.py::test_meta_schedule_feature_extractor_as_string
• test_meta_schedule_measure_callback.py::test_meta_schedule_measure_callback_as_string

Rather than restoring the old behavior, this PR removes the mechanism entirely: the string it produced is just a repr, and tvm-ffi reflection already provides an auto-generated default repr for every object. Keeping a dedicated Python → FFI → C++ callback chain alive only to reproduce that is not worth the complexity.

[gemini-code-assist]

Code Review

This pull request removes the AsString and __str__ methods across several Meta Schedule components (such as CostModel, FeatureExtractor, MeasureCallback, Mutator, Postproc, and ScheduleRule) in both C++ and Python, along with their associated tests and utility functions. The review feedback points out a copy-paste typo in the docstring of CostModel::PyCostModel that incorrectly refers to a "feature extractor" instead of a "cost model".

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[gemini-code-assist]

There is a copy-paste typo in the docstring for CostModel::PyCostModel. It refers to a "feature extractor" instead of a "cost model". Please update it to refer to "cost model". Note that the first line of this docstring (which is outside the diff hunk) also contains "Create a feature extractor..." and should be updated to "Create a cost model..." as well.

   * \return The cost model created.