Revert "Optimize Add/RemoveRolesAsync methods (#2614)" (#2680)

This reverts commit 11481a1.

Author: Misha-133

src/Discord.Net.Rest/Entities/Users/UserHelper.cs

@@ -81,18 +81,14 @@ public static async Task<RestDMChannel> CreateDMChannelAsync(IUser user, BaseDis
 
         public static async Task AddRolesAsync(IGuildUser user, BaseDiscordClient client, IEnumerable<ulong> roleIds, RequestOptions options)
         {
-            await client.ApiClient.ModifyGuildMemberAsync(user.GuildId, user.Id, args: new()
-            {
-                RoleIds = user.RoleIds.Except(new[] { user.Guild.Id }).Concat(roleIds).Distinct().ToArray()
-            }, options);
+            foreach (var roleId in roleIds)
+                await client.ApiClient.AddRoleAsync(user.Guild.Id, user.Id, roleId, options).ConfigureAwait(false);
         }
 
         public static async Task RemoveRolesAsync(IGuildUser user, BaseDiscordClient client, IEnumerable<ulong> roleIds, RequestOptions options)
         {
-            await client.ApiClient.ModifyGuildMemberAsync(user.GuildId, user.Id, args: new()
-            {
-                RoleIds = user.RoleIds.Except(new[] { user.Guild.Id }).Except(roleIds).ToArray()
-            }, options);
+            foreach (var roleId in roleIds)
+                await client.ApiClient.RemoveRoleAsync(user.Guild.Id, user.Id, roleId, options).ConfigureAwait(false);
         }
 
         public static async Task SetTimeoutAsync(IGuildUser user, BaseDiscordClient client, TimeSpan span, RequestOptions options)