Remove alert about needing to install Rosetta on Apple Silicon Mac

[mewfree]

Description of what this PR is changing or adding, and why:
With the release of Flutter 3.0.0 with native Arm64 support on macOS, I believe Rosetta is not necessary anymore.
I was able to install Flutter SDK version 3.0.0 on my M1 MacBook Pro and create a new app, launch a simulator etc... without ever installing Rosetta.

Issues fixed by this PR (if any):

Presubmit checklist

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• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

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[domesticmouse]

PTAL @jmagman

[jmagman]

I only just added this instruction.
Rosetta is still necessary, several of the binaries shipped in the SDK are still x64, like the one that proxies iOS to macOS ports, as well as the gen_snapshot that creates the x64 macOS app release binaries on M1 so users on Intel Macs can install your app.

The most important binaries like dart, macOS and iOS simulator Flutter frameworks are running natively as arm64 though.

[jmagman]

See flutter/flutter#69157

[mewfree]

Thank you for the explanation and link to the other issue to make Flutter 100% ARM native on macOS!

[SametSahin10]

Do we still need to install Rosetta on Apple Silicon machines?

[jmagman]

Yes, you can follow along at flutter/flutter#69157.