Remove outdated installation info about Rustup on Windows

[apiraino]

Closes #659

cc: @rami3l do you agree with just removing the outdated info or is there anything to add specific for Windows?

Thanks!

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[rami3l]

@apiraino Agreed, I think it should fully become the responsibility of the rustup book. Even if the user is using a tarball, we can prevent repeating ourselves when possible in terms of e.g. MSVC environment setup.

OTOH it'd be nice if we can add a link to the rustup book if that hasn't been done already.

[apiraino]

@rami3l ok added a link to the rustup book for clarity. LMK if it LGTY (looks good to you 🙂 ) or feel free to suggest direct edits.

thanks :)