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/Common/NadekoRandom.cs

@@ -2,7 +2,7 @@
 using System.Security.Cryptography;
 
 namespace NadekoBot.Common;
-// todo minby maxby
+
 public class NadekoRandom : Random
 {
     private readonly RandomNumberGenerator _rng;