Cleaned up some unused/commented code

- Small amount of refactoring
- Separated imagesharp extension to ImageSharpExtensions.cs
- Using .join Ienumerable extension instead of string.join in some places

src/NadekoBot/Modules/CustomReactions/NadekoExpressions.cs

@@ -2,8 +2,6 @@
 
 namespace NadekoBot.Modules.NadekoExpressions;
 
-// todo string.join to .Join(", ")
-
 [Name("Expressions")]
 public partial class NadekoExpressions : NadekoModule<NadekoExpressionsService>
 {