Remove legacy footprint geojsons and unused VRT files

The OL viewer migration replaced the bundled *_footprints_3857.geojson
files with FGB tileindexes served via WFS — these geojsons have had no
references in the codebase since.  The ky_20x20_preview*.vrt files were
left over from the pre-tileindex single-COG-VRT approach.

Removed (5.3 MB total):
  - mapfiles/nj_2020_footprints_3857.geojson (4.7 MB)
  - mapfiles/nj_2020_sample_footprints_3857.geojson (2.7 KB)
  - mapfiles/ky_20x20_preview.vrt (596 KB)
  - mapfiles/ky_20x20_preview_3857.vrt (4.6 KB)
  - mapfiles/ky_20x20_preview_3857_z17.vrt (4.5 KB)

The .gitignore whitelist for the removed geojsons is dropped — only
ky_20x20_tileindex.geojson remains whitelisted because admin/index.html
still uses it as a default footprintUrl for its timing probe.

Sample FGB tileindexes (~15 MB) are kept for now since the running
viewer needs them; a future change can move them to a GitHub Release
+ entrypoint download.

Author: Mark Korver

.gitignore

@@ -17,6 +17,7 @@ mapfiles/*_footprints_3857.geojson
 mapfiles/*_tileindex.geojson
 
 # Bundled sample tile indexes — keep tracked.
-!mapfiles/nj_2020_footprints_3857.geojson
-!mapfiles/nj_2020_sample_footprints_3857.geojson
+# (The OL viewer migration replaced the legacy *_footprints_3857.geojson
+# files with FGB tileindexes served via WFS, so only the geojson tileindex
+# referenced by the admin page is whitelisted now.)
 !mapfiles/ky_20x20_tileindex.geojson