feature: Add MESSAGE_REACTION_REMOVE_EMOJI and RemoveAllReactionsForEmoteAsync (#1544)

* Add event and method

* Simplify convert to IEmote

src/Discord.Net.WebSocket/API/Gateway/RemoveAllReactionsForEmoteEvent.cs

@@ -0,0 +1,16 @@
+using Newtonsoft.Json;
+
+namespace Discord.API.Gateway
+{
+    internal class RemoveAllReactionsForEmoteEvent
+    {
+        [JsonProperty("channel_id")]
+        public ulong ChannelId { get; set; }
+        [JsonProperty("guild_id")]
+        public Optional<ulong> GuildId { get; set; }
+        [JsonProperty("message_id")]
+        public ulong MessageId { get; set; }
+        [JsonProperty("emoji")]
+        public Emoji Emoji { get; set; }
+    }
+}