chore: untrack voices/preferences.json from git

Remove the preferences.json file from git tracking while keeping it locally.
This file contains user-specific effect settings that should not be
committed or shared between installations.

voices/preferences.json

@@ -1,5 +0,0 @@
-{
-  "user_voices": {
-    "122139828182712322": "gibralter_good"
-  }
-}