Remove legacy test_changelog_encoding.py (#360)

The test validated UTF-8 and YAML structure of the deprecated
`changelog_entry.yaml`. That file was retired when the repo migrated
to towncrier `changelog.d/` fragments; all three tests now
unconditionally `pytest.skip` because the file no longer exists, and
any fragment-format validation is already handled by the PR CI's
`Check changelog fragment` step.

Also deletes one more on-tree mention of the old format, so nobody
(human or AI agent) rediscovers it from code search and assumes it's
the current path.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MaxGhenis

changelog.d/remove-legacy-changelog-test.removed.md

@@ -0,0 +1 @@
+Remove `policyengine_uk_data/tests/test_changelog_encoding.py`. It validated UTF-8 and YAML structure of the deprecated `changelog_entry.yaml`, which was retired when the repo migrated to towncrier `changelog.d/` fragments. All three tests now unconditionally `pytest.skip` because the file no longer exists, and any fragment-format validation is already handled by the `Check changelog fragment` CI step.