fix(gatsby-plugin-offline): Drop preload link for json from offline shell

[hupe1980]

Description:

Drop preload link for .json from offline shell

Related Issues:

• fixes [v2] Warning in browser console during gatsby serve (after build) #7713
• fixes react-plugin-offline I got Browser console warning: The resource https://www.example.com/static/d/520/path---offline-plugin-app-shell-fallback-a-30-c5a-NZuapzHg3X9TaN1iIixfv1W23E.json was preloaded using link preload but not used within a few seconds from the window's load event. Please make sure it has an appropriate as value and it is preloaded intentionally. #10672

[DSchau]

Looks great! Not going to block this, so let's merge tomorrow--but had you thought about perhaps adding some type of test (we have some offline plugin tests in e2e-tests/production-runtime) to validate that this stays fixed?

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