Remove "amo/polyfills" from the codebase.

[KevinMind]

Describe the problem and steps to reproduce it:

We currently import 2 global polyfills in amo on both client and server entry points.

• isomorphic-fetch (supported in browsers aligned to our .browserslistrc) (caniuse)
• focus-visible (supported in browsers aligned to our .browserslistrc) (caniuse)

We can Deploy to staging and verify using a browserstack smoketest to verify the app loads correctly.

How to verify

For focus-visible we are using this css selector in our Button component.

To test isomorhpic fetch, we simply verify that client side navigations work with successful loading of data from the server. Virtually all sagas call the fetch api, implemented here.

┆Issue is synchronized with this Jira Task

[theaniketgiri]

hi I want to fix this issue, assign me.

[KevinMind]

Old Jira Ticket: https://mozilla-hub.atlassian.net/browse/ADDFRNT-300