refactor: Remove old validator files after 6-validator refactor

Remove dead code from previous architecture:
- performance-validator.ts → replaced by size-validator.ts
- triggering-validator.ts → replaced by keyword-validator.ts
- integration-validator.ts → replaced by harness-validator.ts

These validators were superseded in the previous commit but remained
as unused code. They were:
- NOT exported from src/index.ts
- NOT imported by src/core/linter.ts
- Had incomplete test coverage (17-63%)

After deletion:
- 422 tests passing (down from 463 - removed 41 old validator tests)
- Cleaner architecture with single source of truth
- No functional changes - backward compat maintained in config schema

Author: d3xter666