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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