revert: channel unions (#3918 69d69f2) (#3978)

* revert: channel unions (#3918 69d69f25b9)

* fix: correct tslint:disable

* revert: keep the GuildChannel#setParent change

Co-authored-by: Crawl <icrawltogo@gmail.com>

src/structures/GuildChannel.js

@@ -576,15 +576,6 @@ class GuildChannel extends Channel {
     return this.permissionsFor(this.client.user).has(Permissions.FLAGS.MANAGE_CHANNELS, false);
   }
 
-  /**
-   * Whether this GuildChannel is a partial
-   * @type {boolean}
-   * @readonly
-   */
-  get partial() {
-    return false;
-  }
-
   /**
    * Whether the channel is viewable by the client user
    * @type {boolean}