fix(export): untrack the 8 raw postcode export files (#1492)

These were committed at tiny placeholder sizes during #1039's initial
exports wiring (#1403), but the export.yml workflow regenerates them
at full size every run — 239 MB json/postcodes.json, 263 MB
xml/postcodes.xml, 171 MB yml/postcodes.yml, 123 MB psql/postcodes.sql,
105 MB sqlite/postcodes.sqlite3, 90 MB sql/postcodes.sql, 73 MB
sqlserver/postcodes.sql — all over GitHub's 100 MB hard limit.

#1490 added .gitignore entries for the same paths but gitignore is
inert against tracked files, so the export PR's git push still failed.
Untrack here so the gitignore actually applies; large compressed
.gz versions continue to ship via GitHub Releases.

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

Author: dr5hn