Bump to v4.12.0.

_stackDelete.js

@@ -1,5 +1,3 @@
-import assocDelete from './_assocDelete';
-
 /**
  * Removes `key` and its value from the stack.
  *
@@ -10,10 +8,7 @@ import assocDelete from './_assocDelete';
  * @returns {boolean} Returns `true` if the entry was removed, else `false`.
  */
 function stackDelete(key) {
-  var data = this.__data__,
-      array = data.array;
-
-  return array ? assocDelete(array, key) : data.map['delete'](key);
+  return this.__data__['delete'](key);
 }
 
 export default stackDelete;