Use In-memory cache instead of custom file cache.

src/Icons/Models/Icon.cs

@@ -1,31 +1,18 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Runtime.Serialization;
-using System.Threading.Tasks;
 
 namespace Icons.Models
 {
     [Serializable]
     public class Icon
     {
-
         public byte[] Image { get; }
 
         public string Format { get; }
-        
-        public DateTime CreatedAt { get; }
 
         public Icon(byte[] image, string format)
         {
             this.Image = image;
             this.Format = format;
-            this.CreatedAt = DateTime.Now;
-        }
-
-        public bool HasNotExpired()
-        {
-            return CreatedAt > DateTime.Now.AddDays(-1);
         }
     }
 }