Stop tracking working_files/player_auth.json

This file was still tracked after the previous commit: it is listed in
.gitignore, and git add skips ignored paths, so `git add -A <path>` silently did
nothing for it. Removing it from the index requires `git rm --cached`.

It contains a live auth_code, player_id and server_url for a different player
("tv-terasa" / TV-acasa 1), so it must not be in the repository. The file is
kept on disk; only the tracking is removed.

The two other credential files (player_auth.json, src/player_auth.json) were
already untracked.

Note for anyone repeating this: these files are gitignored, so `git add` will
never stage their removal. Use `git rm --cached <path>`, and verify with
`git ls-files | findstr player_auth` that nothing remains.

working_files/player_auth.json

@@ -1,10 +0,0 @@
-{
-  "hostname": "tv-terasa",
-  "auth_code": "iiSyZDLWGyqNIxeRt54XYREgvAio11RwwU1_oJev6WI",
-  "player_id": 1,
-  "player_name": "TV-acasa 1",
-  "playlist_id": 1,
-  "orientation": "Landscape",
-  "authenticated": true,
-  "server_url": "http://digi-signage.moto-adv.com"
-}