fileutils: deprecate rake DSL

[ilovezfs]

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superenv and the :ruby requirement make a rake DSL unnecessary.

Fixes #3310.

[MikeMcQuaid]

Needs an addition to Library/Homebrew/compat/formula.rb or Library/Homebrew/compat/fileutils.rb with system "rake", *args but when that's the case: 👍

[MikeMcQuaid]