[Frontend] Cleanup: dead code + style nits (issue #241 bundle A)

[YWHyuk]

Summary

Trivial low-risk subset of the #241 cleanup checklist. 4 files changed, +6 / -75 lines. No semantic changes intended.

Refs #241.

Items in this PR

• Dead code
  • `extension_op.py`: drop `generate_inner_product_matrix` (63-line function, no call sites anywhere in repo).
  • `extension_codecache.py`: simplify `w_offset` block — the `if ... w_offset = kwargs['loop_size'][-1]` branch is followed by an unconditional `w_offset = 0`, making the branch dead.
• Copy-paste
  • `extension_codecache.py`: `TileSizeError` default message was the verbatim copy of `SpadOverflowError`'s ("SPAD overflow occurred."). Distinguished as "Tile size constraint violated.".
• Style
  • `mlir_common.py`: drop duplicate `import sympy` (already imported 5 lines earlier).
  • `mlir_common.py`: `MLIRMultiDimTile.get_nr_dim` return annotation `-> str` → `-> int`.
  • `mlir_scheduling.py`: bare `except:` → `except Exception:` (was swallowing `KeyboardInterrupt`/`SystemExit`).
  • `mlir_scheduling.py`: drop dead `node1 = self.revert_group(node1)` rebind — `revert_group` mutates in place with no return, lvalue was rebinding to `None` immediately before `return True`.

Deferred from #241 (other PRs)

• `sparse_mm_dummy_stonne_outer` `yield` bug — needs call-flow check (PR B).
• `dma_write_counter` dead increment — needs constant substitution (PR B).
• Duplicate `hash_prefix`/`get_write_path` extraction — circular-import safety check (PR C).
• Mutable default args sweep — PR D.
• `print()` → `logger` migration — PR E.
• `FileLock` consolidation, template dedup — skipped per issue notes / too invasive.

Test plan

• `python3 -m py_compile` on all four touched files
• Smoke test on sample tests once CI runs

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