Remove mention of polyfill from docs

[mxdvl]

What does this change?

Remove the mention of the polyfill from the browser support section of the README.

Why is this change needed

This has been removed in the latest version: #242

It’s confusing to see it mentioned in the browser support section.

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[tunetheweb]

The polyfills still exist for now. The #242 just added deprecation notices before we remove them completely (likely in v4)