revert: "feat(Partials): add DMChannel/MessageReaction#fetch()… (#3468)

This reverts commit b0047c424b.

src/structures/DMChannel.js

@@ -56,15 +56,7 @@ class DMChannel extends Channel {
    * @readonly
    */
   get partial() {
-    return this.lastMessageID === undefined;
-  }
-
-  /**
-   * Fetch this DMChannel.
-   * @returns {Promise<DMChannel>}
-   */
-  fetch() {
-    return this.recipient.createDM();
+    return !this.recipient;
   }
 
   /**