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

This reverts commit b0047c424b.

src/util/Constants.js

@@ -287,7 +287,6 @@ exports.ShardEvents = {
  * * CHANNEL (only affects DMChannels)
  * * GUILD_MEMBER
  * * MESSAGE
- * * REACTION
  * <warn>Partials require you to put checks in place when handling data, read the Partials topic listed in the
  * sidebar for more information.</warn>
  * @typedef {string} PartialType
@@ -297,7 +296,6 @@ exports.PartialTypes = keyMirror([
   'CHANNEL',
   'GUILD_MEMBER',
   'MESSAGE',
-  'REACTION',
 ]);
 
 /**