chore: remove fetch polyfill from bundle

BREAKING CHANGE: fetch polyfill has been removed. add your own polyfill
to use SDK in old browsers.

package.json

@@ -62,8 +62,7 @@
   "dependencies": {
     "babel-runtime": "^6.26.0",
     "lodash.debounce": "^4.0.8",
-    "sane-domparser-error": "^0.2.0",
-    "whatwg-fetch": "^2.0.4"
+    "sane-domparser-error": "^0.2.0"
   },
   "devDependencies": {
     "@commitlint/cli": "^7.0.0",

rollup.config.js

@@ -19,7 +19,7 @@ const plugins = [
 ];
 
 // NOTE: don't include external dependencies into esm/cjm bundles
-const external = ['babel-runtime/regenerator', 'lodash.debounce', 'sane-domparser-error', 'whatwg-fetch'];
+const external = ['babel-runtime/regenerator', 'lodash.debounce', 'sane-domparser-error'];
 
 // NOTE: see https://github.com/rollup/rollup/issues/408 to understand why we silences `THIS_IS_UNDEFINED` warnings
 const onwarn = (warning, warn) => {

src/adUnit/helpers/fetch/fetchHtml.js

@@ -1,13 +1,7 @@
-// eslint-disable-next-line import/no-unassigned-import
-import 'whatwg-fetch';
-
 const isValidContentType = (contentType) => {
   const normalisedCT = contentType.toLowerCase();
 
-  return [
-    'text/plain',
-    'text/html'
-  ].some((allowedType) => normalisedCT.includes(allowedType));
+  return ['text/plain', 'text/html'].some((allowedType) => normalisedCT.includes(allowedType));
 };
 
 const fetchHtml = async (endpoint) => {

src/vastRequest/helpers/fetch.js

@@ -1,6 +1,3 @@
-/* eslint-disable filenames/match-exported, import/no-unassigned-import */
-import 'whatwg-fetch';
-
 const fetch = async (endpoint, options = {}) => {
   const defaults = {
     credentials: 'include'

yarn.lock

@@ -10344,10 +10344,6 @@ whatwg-encoding@^1.0.1, whatwg-encoding@^1.0.3:
   dependencies:
     iconv-lite "0.4.24"
 
-whatwg-fetch@^2.0.4:
-  version "2.0.4"
-  resolved "http://registry.npmjs.org/whatwg-fetch/-/whatwg-fetch-2.0.4.tgz#dde6a5df315f9d39991aa17621853d720b85566f"
-
 whatwg-mimetype@^2.1.0, whatwg-mimetype@^2.2.0:
   version "2.3.0"
   resolved "https://registry.yarnpkg.com/whatwg-mimetype/-/whatwg-mimetype-2.3.0.tgz#3d4b1e0312d2079879f826aff18dbeeca5960fbf"