API: Remove fetch polyfill

All browser except IE have supported this for longer than
Pleroma even exists.

package.json

@@ -32,8 +32,7 @@
     "vue-router": "^3.0.1",
     "vue-template-compiler": "^2.6.11",
     "vuelidate": "^0.7.4",
-    "vuex": "^3.0.1",
-    "whatwg-fetch": "^2.0.3"
+    "vuex": "^3.0.1"
   },
   "devDependencies": {
     "karma-mocha-reporter": "^2.2.1",

src/services/api/api.service.js

@@ -1,6 +1,5 @@
 import { each, map, concat, last, get } from 'lodash'
 import { parseStatus, parseUser, parseNotification, parseAttachment } from '../entity_normalizer/entity_normalizer.service.js'
-import 'whatwg-fetch'
 import { RegistrationError, StatusCodeError } from '../errors/errors'
 
 /* eslint-env browser */