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

This reverts commit b0047c424b.

src/structures/User.js

@@ -211,7 +211,7 @@ class User extends Base {
    */
   async createDM() {
     const { dmChannel } = this;
-    if (dmChannel && !dmChannel.partial) return dmChannel;
+    if (dmChannel) return dmChannel;
     const data = await this.client.api.users(this.client.user.id).channels.post({ data: {
       recipient_id: this.id,
     } });