Remove prefetch-src from layouts/partials/csp.html.

[Sammy]

Describe the problem:

prefetch-src CSP is deprecated.

Steps to reproduce:

1. Open the developer console in a recent web browser and you'll find the error message: Unrecognized Content-Security-Policy directive 'prefetch-src'.

How to fix this issue

Remove (delimit .Site.Params.csp.prefetchsrc " ") from layouts/partials/csp.html.

[Luiz F. A. de Prá]

Nice. Easy fixing.

If anyone is willing to help with this one, please send a PR.