Remove polyfill warnings from the FE testing docs

What does this MR do?

Remove out of date warnings about polyfills. We now have all the polyfills we require for most cases and there is a clear way to include missing polyfills without a lengthy discovery and review process. Basically, willy wonka's polyfill factory.

Discussion/confusion on Slack: https://gitlab.slack.com/archives/C0GQHHPGW/p1552312398235900

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer