fix(MessageReaction): fetching a removed partial custom emoji (#3955)

* fix: handling GuildEmoji within fetch()

* refactor: remove unused "private" methods

* refactor: make use of Message#fetch() to patch

src/structures/MessageReaction.js

@@ -101,8 +101,12 @@ class MessageReaction {
    * Fetch this reaction.
    * @returns {Promise<MessageReaction>}
    */
-  fetch() {
-    return this.message.reactions._fetchReaction(this.emoji, true);
+  async fetch() {
+    const message = await this.message.fetch();
+    const existing = message.reactions.cache.get(this.emoji.id || this.emoji.name);
+    // The reaction won't get set when it has been completely removed
+    this._patch(existing || { count: 0 });
+    return this;
   }
 
   toJSON() {