refactor: remove legacy schema wire-name helper

[jterapin]

Summary

Removes the obsolete Schema legacy_wire_name compatibility helper after the migrated consumers no longer need it.

• Migrates Schema document deserialization to the canonical cached wire index.
• Removes legacy_wire_name, its RBS declaration, and focused compatibility coverage.

Merge order

Merge after #367 and #369. GitHub does not allow this existing stacked PR to be retargeted, so it remains based on #365.

Validation

• smithy-schema RBS task: 135 examples, 0 failures.
• git diff --check passed.