brew: remove HOMEBREW_ON_DEBIAN7

[iMichka]

This is not used anymore, we are building on ubuntu 16.04 for our portable packages: https://github.com/Homebrew/homebrew-portable-ruby

• [ ] Have you followed the guidelines in our Contributing document?
• [ ] Have you checked to ensure there aren't other open Pull Requests for the same change?
• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes? Here's an example.
• [ ] Have you successfully run brew style with your changes locally?
• [ ] Have you successfully run brew typecheck with your changes locally?
• [ ] Have you successfully run brew tests with your changes locally?

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[MikeMcQuaid]

Nice cleanup, thanks @iMichka!

[BrewTestBot]

Review period ended.

[iMichka]

@MikeMcQuaid I forgot to merge. What happened to CI here?

[MikeMcQuaid]

@iMichka You need to rebase.

[iMichka]

Done