Remove pad and identity rewrite

[Zhenchao Ni (zhenchaoni)]

Summary

Remove two CGC compatibility workarounds following upstream IX fixes already included in the current compiler version (35c01c26).

Removed workaround	Upstream issue	Fix
eliminate-identity	microsoft/ix#1198 — ONNX Identity fails to lower to Foundry and DXCGC	microsoft/ix#1201 adds native Identity lowering.
Pad-specific constant folding (fold_constant_pad_pads) and the fold-constant-pad-pads compatibility alias	microsoft/ix#1199 — Support constant folding of ONNX Pad parameters before lowering	microsoft/ix#1206 improves Cast/Pad chain support, static shape inference, and lowering type compatibility.

Changes

• Remove the Identity rewrite implementation, capability registration, and public export.
• Remove the Pad-specific folding prepass and its dedicated dead-producer cleanup.
• Remove the fold-constant-pad-pads alias and public function export.
• Update documentation and tests to reflect the retired rules.
• Preserve cgc-constant-folding and its shared integer/boolean expression evaluator. General folding remains necessary for static shape chains and continues to fold Pad parameters in graphs containing Shape.
• Leave compiler options, including the default-enabled topological sorting, unchanged.

Validation

• 203 affected unit tests passed.
• All five models that previously used the Pad-specific prepass retained identical Pad parameter values.
• GPU outputs before and after Pad-prepass removal were elementwise identical across three input samples per model.
• The three SOD models required topological sorting to be disabled for the GPU comparison because the current wheel does not yet include the fix for microsoft/ix#1207. Their default-path conversion failure remains unchanged.
• End-to-end SOD validation through JSON config, build, perf, and eval is still in progress.