Remove deprecated attributeGraphEnabled and swiftToolchainSupported API

[Kyle-Ye]

Summary

• Remove attributeGraphEnabled (replaced by attributeGraphVendor)
• Remove swiftToolchainSupported (was always true)

Test plan

• Verify swift build succeeds
• Verify no remaining references to removed symbols

[augmentcode]

🤖 Augment PR Summary

Summary: Removes deprecated shim APIs (attributeGraphEnabled, swiftToolchainSupported) in favor of the single attributeGraphVendor entry point.
Changes: Deletes the deprecated exported constants across all build configurations in GraphShims.swift.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.08%. Comparing base (6c04001) to head (0b402d5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #218   +/-   ##
=======================================
  Coverage   32.08%   32.08%           
=======================================
  Files          70       70           
  Lines        2596     2596           
=======================================
  Hits          833      833           
  Misses       1763     1763           

Flag	Coverage Δ
ios	32.10% <ø> (ø)
macos	9.95% <ø> (ø)
ubuntu	0.00% <ø> (ø)

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