brew.rb: Undeprecate HOMEBREW_BUILD_FROM_SOURCE

[RandomDSdevel]

• 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? (Partly, see my commit message.)
• 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 tests with your changes locally? (All tests pass locally except 'test/os/mac_spec.rb,' which fails with an unrelated error stating that, "'Locale::parse' failed to parse 'OS::Mac::language'.")

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     I'm leaving this open for the time being instead of merging it immediately, at least until we formally decide on how to proceed with #2.

[RandomDSdevel]

     I'm keeping this as rebased against the latest upstream commits as I can while #2 is still pending, though I'm also tracking developments there in terms of what branch this PR is against.

[RandomDSdevel]

     Almost/sort-of/kind-of/nearly forgot to come back and merge this PR now that we've settled on a branch-usage workflow in #2.