refactor: remove unused code

Author: galaxian85

lib/common/utils.js

@@ -289,10 +289,6 @@ function normalizeReference(str) {
   return str.toLowerCase().toUpperCase();
 }
 
-/* eslint-env browser */
-var _g = typeof global !== 'undefined' ? global : window;
-var tokensRef = new _g.WeakMap();
-
 function getLineOffset(state) {
   if (state.env.parentToken.parentType === 'blockquote') {
     const blockState = state.env.state_block;