fix: revert named exports

lib/axios.js

@@ -14,7 +14,6 @@ import toFormData from './helpers/toFormData.js';
 import AxiosError from './core/AxiosError.js';
 import spread from './helpers/spread.js';
 import isAxiosError from './helpers/isAxiosError.js';
-import AxiosHeaders from './core/AxiosHeaders.js';
 
 /**
  * Create an instance of Axios
@@ -74,6 +73,4 @@ axios.formToJSON = thing => {
   return formDataToJSON(utils.isHTMLForm(thing) ? new FormData(thing) : thing);
 };
 
-axios.AxiosHeaders = AxiosHeaders;
-
 export default axios