Revert "fix(Structures): allow multiple extensions by checking protot…

…ype chain (discordjs#3217)"

This reverts commit 33ede42.

src/util/Structures.js

@@ -55,8 +55,8 @@ class Structures {
       throw new TypeError(`The extender function must return the extended structure class/prototype ${received}.`);
     }
 
-    if (!(extended.prototype instanceof structures[structure])) {
-      const prototype = Object.getPrototypeOf(extended);
+    const prototype = Object.getPrototypeOf(extended);
+    if (prototype !== structures[structure]) {
       const received = `${extended.name || 'unnamed'}${prototype.name ? ` extends ${prototype.name}` : ''}`;
       throw new Error(
         'The class/prototype returned from the extender function must extend the existing structure class/prototype' +