Reverts #57066 (SERVER-130150), which emitted a post-merge-review JSON blob from the PR approval-recency action, while preserving the SPDX license header applied later by #58311 (SERVER-127760).

1. 1. Changes

`validate_pr_approval_recency.py`

• Removed the `POST_MERGE_REVIEW_JSON` marker and `_build_review_json` helper
• Removed the `json` import and the `post_approval_changed_file_paths` metric
• Restored `write_logfmt(buildevent_file, metrics)` (dropped the list-filtering wrapper)

`tests/unit/test_metrics.py`

• Removed the test cases covering the reverted JSON-blob behavior

The revert auto-merged cleanly around the SPDX header — the header and the reverted logic do not overlap, so #58311's `# SPDX-License-Identifier: SSPL-1.0` header is retained.

1. 1. Testing

Action unit tests pass (`21 passed`).