multirust: removed deprecated formula

[HarryLovesCode]

Removed multirust as it has been deprecated and is no longer supported.

Multirust repository

• Removed formula
• Removed reference to formula as conflict

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• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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[fxcoudert]

How about providing rustup, and migrating multirust users to rustup? Would that make sense, from a user perspective? I.e. are the two tools sufficiently close that a user could transition without effort?

[JCount]

There has been a previous discussion about adding a rustup formula to Homebrew, so if you would like to try and migrate multirust users the way @fxcoudert suggested, you should read #9617

Even without the addition of a rustup formula, this should probably be merged, given multirust's deprecated state.

[alex]

Given that multirust was deprecated in favor of rustup, it seems a bit unfortunate to remove this before the replacement is available.