refactor(bot): remove saving own user ID from ready event, rely on ID in config

PluralKit.Bot/BotConfig.cs

@@ -5,7 +5,7 @@ public class BotConfig
     public static readonly string[] DefaultPrefixes = { "pk;", "pk!" };
 
     public string Token { get; set; }
-    public ulong? ClientId { get; set; }
+    public ulong ClientId { get; set; }
 
     // ASP.NET configuration merges arrays with defaults, so we leave this field nullable
     // and fall back to the separate default array at the use site :)