Remove "extends IHostedService" from classes where it's not required (#236)

Originally, these classes were services because I thought that all
DI-resolvable classes need to be services. However, this is not true, so
we can make these classes (notably Utility and GuildDataService) not
extend anything. `UtilityService` was renamed to `Utility` for
simplicity

---------

Signed-off-by: Octol1ttle <l1ttleofficial@outlook.com>

src/Commands/MuteCommandGroup.cs

@@ -32,11 +32,11 @@ public class MuteCommandGroup : CommandGroup
     private readonly IDiscordRestGuildAPI _guildApi;
     private readonly GuildDataService _guildData;
     private readonly IDiscordRestUserAPI _userApi;
-    private readonly UtilityService _utility;
+    private readonly Utility _utility;
 
     public MuteCommandGroup(
         ICommandContext context, GuildDataService guildData, IFeedbackService feedback,
-        IDiscordRestGuildAPI guildApi, IDiscordRestUserAPI userApi, UtilityService utility)
+        IDiscordRestGuildAPI guildApi, IDiscordRestUserAPI userApi, Utility utility)
     {
         _context = context;
         _guildData = guildData;