optimize<Preference>: remove avater server

src/Models/AvatarServer.cs

@@ -1,22 +0,0 @@
-using System.Collections.Generic;
-
-namespace SourceGit.Models {
-
-    /// <summary>
-    ///     支持的头像服务器
-    /// </summary>
-    public class AvatarServer {
-        public string Name { get; set; }
-        public string Url { get; set; }
-
-        public static List<AvatarServer> Supported = new List<AvatarServer>() {
-            new AvatarServer("Gravatar", "https://www.gravatar.com/avatar/"),
-            new AvatarServer("Gravatar - 极客族", "https://sdn.geekzu.org/avatar/"),
-        };
-
-        public AvatarServer(string name, string url) {
-            Name = name;
-            Url = url;
-        }
-    }
-}

src/Models/Preference.cs

@@ -58,11 +58,6 @@ namespace SourceGit.Models {
                 set => FontFamilyContentSetting = value;
             }
 
-            /// <summary>
-            ///     头像服务器
-            /// </summary>
-            public string AvatarServer { get; set; } = "https://www.gravatar.com/avatar/";
-
             /// <summary>
             ///     是否启用深色主题
             /// </summary>