docs(sra): drop the id-identity impl documents

Remove docs/impl/001-id-identity-on-mirror.md and
002-id-identity-test-plan.md — the design decision (S13) and the test
plan are already consolidated in sra-design.md (§3.2 S13, §4.3.5); the
separate impl documents were redundant with the main design doc.

Author: LinZexiao